PRESSURE SETTING OF SAFETY VALVES Before attempting to adjust the safety valves of any boiler,its essential to verify the accuracy of the boiler pressure gauge,safety valves must be set at a pressure not exceeding 3% above the approved working pressure each safety valve is adjusted independently. PROCEDURE FOR SETTING SAFETY VALVE
- Use gagging tool to prevent simultaneous lifting of valves at set pressure
- Remove the caps and keep the valve spindle on valve gagged, set the other valve first.
- Keep the compression nut screwed down,on the valve to be set, till the previous marks.
- Raise steam slowly and bring to set pressure
- You will hear a hissing sound, this is the floating condition of the valve and subsequently the valve will lift. if the valve lifts before the set pressure is achieved tighten the compression nut. if the valve fails to lift at the set pressure loosen the compression nut, maintain the boiler pressure constant while adjusting, reduce boiler pressure, remove gagging tool and fit to other valve for setting.
- After both valves are set, remove the gagging tool and fire the boiler to maximum capacity. note full flow of waste steam from waste steam pipe. note pressure on gauge, the pressure should not exceed 10% of set pressure. after completion refit caps, cotter, locks and easing gear.
- ACCUMULATION TEST
- It is the problem associated with spring loaded safety valve.
- When valve is lifted >spring got lifted>its set pressure increases
- Hence further lift of safety valve required high pressure.
- This increase in pressure is called ACCUMULATION OF PRESSURE
- Carried out on safety valves during installation of boiler.this is a classification society requirement to test the capacity of safety valve.
- Tests are carried out with steam stop valves shut and boiler firing fully.
- The safety valve should be able to maintain the accumulation pressure within 10% of the working pressure.
- Smoke tube boilers tested for 15 minutes
- Water tube boilers tested for 7 minutes
- Accumulation tests can be waived off for water tube boilers as it would damage the superheaters.
: PRESSURE SETTING OF SAFETY VALVES
- 1 What pressure should a boiler safety valve be set at?
- 2 At what PSI is the safety valve set to open?
- 3 What pressure level is safety valve set to open?
How do you adjust a boiler safety valve?
Boiler Safety Valve Setting- Step Wise Procedure – 1) Make sure that tools required for the setting like spanner, gagging tool etc are ready.2) Boiler is ready to be fired. Make sure that the steam blow off line after the safety valve, have a drain and it should be unclogged and should have free passage.3) Make sure that safety valve have been assembled properly.4) Close the main steam stop valve.5) The other safety valve should be gagged to prevent it from lifting.
It will set later on when the first safety valve have set.6) Start firing the boiler manually and slowly raise the steam pressure.7) Screw down the compression nut little bit more than the previous distance. If you don’t have previous reading, then it will require little bit more attempts before you can actually set the valve.8) Now let’s say you have to set valve at 9 bar.
So, raise the pressure slowly. Just before the 9 bar, like 8.8 or 8.9, you will start seeing little bit of steam coming out of the safety valve. Note: If you safety valve lifts at 10 bar instead of 9, then don’t try to set it down at the same time. Lower the pressure to 7 bar or something and then adjust the compression nut and then again raise the pressure to 9 bar.
Otherwise you get all the wrong adjustment.9) Once you are sure that it lifts at 9 bar, then 10) The distance between the bottom of the compression nut and the top of the safety valve cover bushes are measured by a scale of vernier caliper.11) Then we have to make split ring of the same size and put.
This kind of locks the compression nut.12) Again fire the boiler and remove the gagging tool from the second safety valve and put it on the first safety valve.13) Repeat the process with the second safety valve similar to the first one.14) After both valves are set, they usually have some difference like 0.3 bar.
What pressure should a boiler safety valve be set at?
When I teach my steam classes, I ask the attendees, “Do you test the pop safety valve?” Most do not. When I ask why, they tell me the same reason; the safety valve will leak. I joke during the classes that you do not want to test the pop safety valve on a Friday afternoon because it will almost certainly leak.
I then ask, Do you check the low water cutoff? They look at me like I have a third eye and say they always check the low water cutoff. If you test the low water cutoff, you should test the pop safety valve. It is the last line of defense against a potential catastrophe. One of the things I do when performing a boiler service call is to explain the duty of the pop safety valve and ask the customer if they would like to have it tested.
I explain that it could leak and if they refuse to test it, I will notate it on my service call in case something happens. In this way, my company is protected. Understand the pop safety valve The best way to understand the pop safety valve is to read the instructions which came with the valve.
I don’t have a life, and while you are watching the Masked Singer, I read O & M manuals. I know, I’m weird. I figure it’s my job to share things I find while reading these page-turners. The manufacturer hides all sorts of useful tidbits on the installation and maintenance of their valve. I have enclosed some information I gleaned while reading the instructions for a Conbraco/Apollo pop safety valve.
Installation The valve must be mounted in a vertical, upright position directly to a clean, tapped opening in the top of the boiler. I see many safety valves installed horizontally and wonder if that voids the warranty. There should be no restrictions or valves in the piping to or from the safety valve.
The installation instructions require the discharge piping to be schedule 40 pipe. They specifically say not to use schedule 80 pipe, which is 50% thicker than schedule 40 pipe. Many installers use copper tubing for the discharge, which does not meet the instructions. The other thing which confuses me the manufacturer instructs you not to use a pipe wrench to install the safety valve.
I would wager 99% of all valves are installed using a pipe wrench. I wonder what kind of valve they want you to use. How often should it be tested? I consult the pop safety manufacturer or the building insurance company to determine the frequency of tests.
- Apollo recommends quarterly testing using the Try Lever Test unless the valve is located in a severe service condition, and then it should be done more often.
- They further state the pop safety valve should have a Pressure Test annually before the heating season or at the end of any non-service period.
This test will check your courage as you have to jump out the pressure controls and watch the operation of the boiler as the pressure builds. If the pop safety valve opens at the set pressure, the valve is working properly. This is not a test a novice should do alone.
How to test the safety valve Apollo suggests checking the pop safety valve at or near the maximum operating pressure by holding the test lever fully open for at least 5 seconds and letting it pop closed. On a low-pressure steam system, the pop safety valve is set for 15 psi. I like to run the boiler steam pressure up to 12 psi or higher to check the pop safety valve.
After the test, I drop it to the operating pressure the owner requires. If the valve does not open, the boiler should be shut down until it is checked by a licensed contractor or qualified service person. Pressure margin The pop safety manufacturer requires a minimum pressure differential of five psi between the pressure relief valve set pressure and the boiler operating pressure.
It further states, Under no circumstances should the margin be less than five psig. On a low-pressure steam boiler, the pop safety valve will be set for 15 psi. That means the boiler steam pressure should be ten psi or lower. In breweries, it is common to see the boiler pressure set at 12-14 psi. This is less than the five psi differential and could create a dangerous condition.
Be sure you feel comfortable testing the pop safety valve and read the instructions. If you would like to know more about maintaining steam boilers, please check out my books on Amazon or visit my website, BrewingwithSteam.com
How does a safety valve work on a boiler?
Safety Valves: What, Why, and How 3 minutes reading time (587 words) Years ago, it was not uncommon to read news about tragic boiler explosions, sometimes resulting in mass destruction. Today, boilers are equipped with important safety devises to help protect against these types of catastrophes. Let’s take a look at the most critical of these devices: the safety valve. What The safety valve is one of the most important safety devices in a steam system. Safety valves provide a measure of security for plant operators and equipment from over pressure conditions. The main function of a safety valve is to relieve pressure. It is located on the boiler steam drum, and will automatically open when the pressure of the inlet side of the valve increases past the preset pressure.
- All boilers are required by ASME code to have at least one safety valve, dependent upon the maximum flow capacity (MFC) of the boiler.
- The total capacity of the safety valve at the set point must exceed the steam control valve’s MFC if the steam valve were to fail to open.
- In most cases, two safety valves per boiler are required, and a third may be needed if they do not exceed the MFC.
There are three main parts to the safety valve: nozzle, disc, and spring. Pressurized steam enters the valve through the nozzle and is then threaded to the boiler. The disc is the lid to the nozzle, which opens or closes depending on the pressure coming from the boiler.
- The spring is the pressure controller.
- As a boiler starts to over pressure, the nozzle will start to receive a higher pressure coming from the inlet side of the valve, and will start to sound like it is simmering.
- When the pressure becomes higher than the predetermined pressure of the spring, the disc will start to lift and release the steam, creating a “pop” sound.
After it has released and the steam and pressure drops below the set pressure of the valve, the spring will close the disc. Once the safety valve has popped, it is important to check the valve to make sure it is not damaged and is working properly. Why A safety valve is usually referred to as the last line of safety defense.
Without safety valves, the boiler can exceed it’s maximum allowable working pressure (MAWP) and not only damage equipment, but also injure or kill plant operators that are close by. Many variables can cause a safety valve on a boiler to lift, such as a compressed air or electrical power failure to control instrumentation, or an imbalance of feedwater rate caused by an inadvertently shut or open isolation valve.
Once a safety valve has lifted, it is important to do a complete boiler inspection and confirm that there are no other boiler servicing issues. A safety valve should only do its job once; safety valves should not lift continuously. Lastly, it is important to have the safety valves fully repaired, cleaned and recertified with a National Board valve repair (VR) stamp as required by local code or jurisdiction.
- Safety valves are a critical component in a steam system, and must be maintained.
- All of Nationwide Boiler’s include on to two safety valves depending on the size; one set at design pressure and the other set slightly higher than design.
- By request, we can reset the safeties to a lower pressure if the application requires it.
In addition, the valves are thoroughly checked after every rental and before going out to a new customer, and they are replaced and re-certified as needed. : Safety Valves: What, Why, and How
What is the setting of a safety valve?
Safety Valve 101 – North American Safety Valve Industries, Inc. A little product education can make you look super smart to customers, which usually means more orders for everything you sell. Here’s a few things to keep in mind about safety valves, so your customers will think you’re a genius.
A safety valve is required on anything that has pressure on it. It can be a boiler (high- or low-pressure), a compressor, heat exchanger, economizer, any pressure vessel, deaerator tank, sterilizer, after a reducing valve, etc. A safety valve usually refers to a valve that will see steam, air, or gas, and a relief valve is used on liquid.
Some valves do both. There are four main types of safety valves: conventional, bellows, pilot-operated, and temperature and pressure. For this column, we will deal with conventional valves. A safety valve is a simple but delicate device. It’s just two pieces of metal squeezed together by a spring.
It is passive because it just sits there waiting for system pressure to rise. If everything else in the system works correctly, then the safety valve will never go off. Why set pressure is higher than operating pressure A safety valve is NOT 100% tight up to the set pressure. This is VERY important. A safety valve functions a little like a tea kettle.
As the temperature rises in the kettle, it starts to hiss and spit when the water is almost at a boil. A safety valve functions the same way but with pressure not temperature. The set pressure must be at least 10% above the operating pressure or 5 psig, whichever is greater.
- So, if a system is operating at 25 psig, then the minimum set pressure of the safety valve would be 30 psig.
- Most valve manufacturers prefer a 10 psig differential just so the customer has fewer problems.
- If a valve is positioned after a reducing valve, find out the max pressure that the equipment downstream can handle.
If it can handle 40 psig, then set the valve at 40. If the customer is operating at 100 psig, then 110 would be the minimum. If the max pressure in this case is 150, then set it at 150. The equipment is still protected and they won’t have as many problems with the safety valve.
Here’s another reason the safety valve is set higher than the operating pressure: When it relieves, it needs room to shut off. This is called BLOWDOWN. In a steam and air valve there is at least one if not two adjusting rings to help control blowdown. They are adjusted to shut the valve off when the pressure subsides to 6% below the set pressure.
There are variations to 6% but for our purposes it is good enough. So, if you operate a boiler at 100 psig and you set the safety valve at 105, it will probably leak. But if it didn’t, the blowdown would be set at 99, and the valve would never shut off because the operating pressure would be greater than the blowdown.
- All safety valves that are on steam or air are required by code to have a test lever.
- It can be a plain open lever or a completely enclosed packed lever.
- Make sure your customers get the right size Safety valves are sized by flow rate not by pipe size.
- If a customer wants a 12″ safety valve, ask them the flow rate and the pressure setting.
It will probably turn out that they need an 8×10 instead of a 12×16. Safety valves are not like gate valves. If you have a 12″ line, you put in a 12″ gate valve. If safety valves are sized too large, they will not function correctly. They will chatter and beat themselves to death.
- The lower the pressure setting and the higher the flow rate, the larger the valve required.
- Safety valves need to be selected for the worst possible scenario.
- If you are sizing a pressure reducing station that has 150 psig steam being reduced to 10 psig, you need a safety valve that is rated for 150 psig even though it is set at 15.
You can’t put a 15 psig low-pressure boiler valve after the reducing valve because the body of the valve must to be able to handle the 150 psig of steam in case the reducing valve fails. Debris + seating surface = leaks The seating surface in a safety valve is surprisingly small.
In a 3×4 valve, the seating surface is 1/8″ wide and 5″ around. All it takes is one pop with a piece of debris going through and it can leak. Here’s an example: Folgers had a plant in downtown Kansas City that had a 6×8 DISCONTINUED Consolidated 1411Q set at 15 psig. The valve was probably 70 years old.
We repaired it, but it leaked when plant maintenance put it back on. It was after a reducing valve, and I asked him if he played with the reducing valve and brought the pressure up to pop the safety valve. He said no, but I didn’t believe him. I told him the valve didn’t leak when it left our shop and to send it back.
- When it came back, I laid it down on the outlet flange and looked up the inlet.
- There was a 12″ welding rod with the tip stuck between the seat and the disc.
- That rod was from the original construction and didn’t get blown out properly and just now it got set free.
- The maintenance guy didn’t believe me and came over and saw it for himself (this was before cell phones when you could take a picture).
Troubleshooting: Some questions to ask If there is a problem with a safety valve, 99% of the time it is not the safety valve or the company that set it. There may be other reasons that the pressure is rising in the system before the safety valve. Some ethanol plants have a problem on starting up their boilers.
The valves are set at 150 and they operate at 120 but at startup the pressure gets away from them and there is a spike, which creates enough pressure to cause a leak until things get under control. If your customer is complaining that the valve is leaking, ask questions before a replacement is sent out.
What is the operating pressure below the safety valve? If it is too close to the set pressure then they have to lower their operating pressure or raise the set pressure on the safety valve. Is the valve installed in a vertical position? If it is on a 45-degree angle, horizontal, or upside down then it needs to be corrected.
I have heard of two valves that were upside down in my 47 years. One was on a steam tractor and the other one was on a high-pressure compressor station in the New Mexico desert. He bought a 1/4″ valve set at 5,000 psig. On the outlet side, he left the end cap in the outlet and put a pin hole in it so he could hear if it was leaking or not.
He hit the switch and when it got up to 3,500 psig the end cap came flying out like a missile past his nose. I told him to turn that sucker in the right direction and he shouldn’t have any problems. I never heard from him so I guess it worked. Check the outlet piping If the set pressure is correct, and the valve is vertical, ask if the outlet piping is supported by something other than the safety valve.
- If they don’t have pipe hangers or a wall or something to keep the stress off the safety valve, it will leak.
- There was a plant in Springfield, Mo.
- That couldn’t start up because a 2″ valve was leaking on a tank.
- It was set at 750 psig, and the factory replaced it 5 times.
- We are not going to replace any valves until certain questions are answered.
I was called to solve the problem. The operating pressure was 450 so that wasn’t the problem. It was in a vertical position so we moved on to the piping. You could tell the guy was on his cell phone when I asked if there was any piping on the outlet. He said while looking at the installation that he had a 2″ line coming out into a 2×3 connection going up a story into a 3×4 connection and going up another story.
I asked him if there was any support for this mess, and he hung up the phone. He didn’t say thank you, goodbye, or send me a Christmas present. Pipe dope is another problem child. Make sure your contractors ease off on the pipe dope. That is enough for today, class. Thank you for your patience. And thank you for your business.
: Safety Valve 101 – North American Safety Valve Industries, Inc.
Can a safety valve be adjusted?
The safety valve is then adjusted using special tools to calibrate the mechanism to the specifications of the system, the performance of the water heater and the legal requirements.
At what PSI is the safety valve set to open?
Most air brake safety valves are set to release excess pressure from the system at 150 psi (1035 kPa).
What is healthy boiler pressure?
The water pressure inside a boiler system should usually be somewhere between 1 and 1.3 bar; check your manual to find out its optimum range. If the boiler pressure is too high or low and falls outside this range, your central heating won’t work efficiently – that’s the range at which all the components are designed to operate.
What pressure level is safety valve set to open?
Explanation The safety valve protects air tanks and the rest of an air brake system from accumulating too much air pressure. The safety valve is usually set to open at 150 psi. This questions appears in the following tests:
How does this safety valve work?
Figure 1: Safety valve A safety valve protects a system against overpressure. Overpressure occurs when the system’s pressure exceeds the Maximum Allowable Working Pressure (MWAP) or the pressure for which the system is designed. Safety valves can open very quickly compared to relief valves,
- A safety valve opens from a set pressure; the valve first opens a little, then opens fully so that the unwanted pressure is removed from the system as quickly as possible.
- Safety valves prevent pressure increases that lead to malfunctions, fire hazards, or explosions.
- The system’s media fully actuates a safety valve, keeping it working in a power failure.
Safety valves only have mechanical parts, which operate when electronic or pneumatic safety devices fail.
What is set and reset pressure for safety valve?
Pressure Relief Valve: This device is generally fitted on liquid lines like water, oil line. In this, valve the opening is proportional to increase in the line or vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually.
In relief valve valves may not open 100%, as the line pressure reduces valves closes gradually. Pressure relief valves have higher flow capacities Pressure Safety Valve: It is fitted on compressible fluid or gas lines. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.
Pressure safety valve & relief valves are used for system, equipment & man power protection. Pressure reducing valve: These may be of hydraulic or pneumatic type used for water lines. This valve reduces the pressure of the water that goes through it, and is used to obtaining a regulated and constant value at its outlet.
Pressure control valves: These may be of hydraulic or pneumatic type used for steam lines Safety valve: A safety valve must always be sized and able to vent any source of steam so that the pressure within the protected apparatus cannot exceed the maximum allowable accumulated pressure.Here the valves sizing, manufacturing, installation, positioning & setting are more important.
Factors to be considered for selection/design of a pressure safety valve:
Connection size and type
Blow down & Operating gap
Terminology used in safety valves: Set pressure: It is the pressure at which safety valve lifts or pops up.It is usually 106-107% of operating pressure. Reseat pressure: It is the pressure at which Safety valve seats. Blowdown: It is the Blowdown is the difference between set pressure and reseating pressure of a safety valve expressed as a percentage of set pressure.
Blow down of safety valve = (Set pressure – Reseat pressure) X 100 / Set pressure Blow down of safety valves is in the range of 2 to 5%. Chattering: Excessive pressure loss at the inlet of the safety valve will cause extremely rapid opening and closing of the valve, this is called as chattering. Chattering may result into lowered capacity as well as damage to the seating surface of the valve.
Continuous chattering may result into damage to the other parts. Following recommendation wil assists in eliminating the chattering
The area of the inlet nozzle should be equal to the the inlet area of safety valve & that nozzle should be short as possible
Inlet nozzle corners must be rounded to a radius of not less than ¼ of the diameter opening
Sonic Vibrations: Flashing, choked flow sudden flow or cut/off of steam in safet valves & related lines may result into sonic vibration. This velocity is usually reached when the valve pressure drop rises to 50% of the upstream pressure. Vibration of long pipelines can also occur due to mechanical damage. Precautions to avoid sonic vibrations:
Safety valve should be installed at least 8D to 10D of pipe diameter down stream from any bend in steam line.
Safety valves should not be installed closer than 8D to 10D to pipe diameters either upstream or down stream from the diverging or converging “Y” fittings.
The safety valve nozzle should never be installed in a steam line in a position directly opposite a branch line of equivalent size.
Accumulating test pressure: The accumulation test is done on boilers to limit the excessive pressure rising while the safety valve is in open. The test is carried on new boilers or new safety valves with full firing condition with MSSV and feed water valves closed.
Exhaust drain & cover plate vent piping must be installed so that they will not impose under pressure on the safety valve.
Note: Do not plug the cover plate hole or do not reduce the hole piping size.
Discharge pipe of the safety valve should not be supported on the valve body
Clearance between the valve exhaust piping and the discharge stack should be sufficient to prevent contact when considering thermal expansion of the boiler valve
Steam flowing vertically out of the discharge elbow produces a downward reaction on the elbow, in proportion on the quantity of steam flowing & its velocity. In no case should discharge piping smaller than the outlet valve For optimum performance safety valves should be serviced regularly
Valve assembly should be within 1 0 vertical alignments.
Gaskets fitted should be of correct size, should not close the valve inlet opening
Adjustment of set pressure: Safety valves are set +/- 1% of set pressure.Set pressure should not be changed without the permission of manufacturing unit. Before proceeding to check the popping (lift) pressure, ensure the pressure gauges used are calibrated.
To adjust the popping pressure, remove the lifting gear, exposing the adjusting bolt lock nut. Loosen the lock nut if the opening pressure is low tighten (turn clockwise) the adjusting bolt, if it is high loosen (turn counter clockwise) the bolt. After each adjustment the lock nut should be securely tightened to prevent loosening of the bolt.
Adjustment of blowdown: If the blow down is not as desired when the set pressure has been obtained, it is must to adjust the rings. The guide (adjusting) ring is the principal blow down control ring. To change its position, remove the guide set screw on the back of the valve body.
- Insert a screw driver or similar tool and engage one of the notches (these can be seen through set screw hole).
- The ring can then be turned to the right or left as desired.
- Turning the guide (upper) ring to the right raises it up and reduces the blow down.
- Turning the guide (upper) ring to the left lowers it and increases the blow down.
After each adjustment always replace and tighten the set screw being careful that its point engages a notch and does not rest on the top of the tooth. Note: Do not attempt to adjust blow down with lower ring Factors which cause safety valve to damage or failure:
Quantity & quality of the steam
Discharge piping stress and back pressure
Variation in ambient temperature
Improper bolting of flanges
Foreign material in the steam
Improper method of assembly & disassembly
Guidelines for Boiler safety valve setting: Preliminary checks
Ensure calibrated pressure & temperature gauges are fitted.
Gauges for each individual valves should be fitted
Discharge piping has to be inspected for binding on the valves,supports and welds on piping.
A rope appx.6-7 meters with a hook one end should be attached to the valve lifting lever before starting the pressure rise. It will help in operating the lever to avoid chattering & over pressure
Have the correct tooling available
Establish the good communication system
If the unit has Electromatic safety valve, this valve should be in operation firts for more safety of the unit.
Drum valves to should be tested first: Possibilities of valve part damage because of GIRL BLASTING are grater on superheated valves in contrast to the drum valves,If super heater valve is gagged after seat damage while testing other valves, the total valve damage will increase. Boiler temperature increases during the testing cycle of the Drum valves. Consequently higher temperature steam will be available for super heater steam valves and produce accurate results
Keep water level low as possible, if drum level is high the safety valves may slugged with water causing long blow down & also may result damages to seat & disc.
Maintain pressure rising in the range of 2-3 kg/cm2 per minute, slow pressure rising may result into simmering of the valve.
If fuel feeding system fails at nearer set pressure, then reduce the boiler pressure at least 10% & raise again. Holding the boiler pressure nearer to set pressure for long time may result into simmering & valve lift erratically.
If a valve has to be lifted several times, cooling off period is very must. Cooling period is around 20-30 minutes.
If valves have not been tested with hydro test prior to the steam condition, it is recommended to hand lift before steam actuation.
Safety valve floating procedure:
Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valves are gagged.
Start the boiler as per cold start up procedure by modulating the firing. When the drum pressure reaches about 60–70% of operating pressure gently tighten gage on other safety valve.
Raise pressure slowly by throttling start up vent valve. When 80% of popping up pressure is reached manually operate the safety valve under test. This will blow off any debris or dust left over in the valve internals.
Raise the boiler pressure by modulating the firing
When the pressure reaches nearer to the set pressure close the start up vent. While the safety valve pops (lift), open the start up vent valve and note down the lifting/set pressure value.When the valve sits back, note down the reset pressure
Control of drum level is important to avoid possibility of water carry over from drum to the super heater.
The set pressure is adjusted by either tightening or loosening the adjusting nut. Tightening the nut increases the set pressure and vice versa
Blow down is adjusted by upper rings adjustment. After setting the set pressure and blow down, bring down the boiler pressure to operating level.
Examples-1: A boiler steam drum safety valve lifts at 125 kg/cm 2 and reseats at 120 kg/cm 2, then calculate its blow down percentage? BD% = (125 – 120) X 100/125 =4.0% Example-2: A boiler super heater safety valve has blow down 3% & has been set at 70 kg/cm2, calculate the reseat pressure.3% = (70-P2) X 100/70 Reseat pressure P2 =67.90 kg/cm2 Precautions shall be taken during Super heater safety valve set at lower operating temperature than actual: Safety valves blow down should be set more than required, as blow down percentage decreases as the steam temperature increases.
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What pressure level is safety valve set to open?
Explanation The safety valve protects air tanks and the rest of an air brake system from accumulating too much air pressure. The safety valve is usually set to open at 150 psi. This questions appears in the following tests: