PRV ON Boiler Unable to Lift at Set Pressure

If some other PRVs were relieving into a common discharge header at the time, there may have been back pressure in the header. (This could happen if the header was too small, for instance.)

Some types of PRVs are sensitive to upstream pressure only; others are sensitive to both upstream and downstream pressure.

Thanks but in our case, we are venting into atmosphere.... Its a spring mounted valve and i would assume spring force on the disc is the only back pressure, am i right?

In that case, it sounds as though you are right.

Another possibility to consider is that the pressure gauge may have been inaccurate. After the PRV failed to lift, was it then tested again with a calibrated gauge?

Did the PRV discharge directly to atmosphere, or was there a vertical discharge pipe that might have become filled with rain or condensate?

Once Pressure transmtter reported the pressure surge, it was attended immediately and controls were put in place to reduce it. It wasn't an issue with gauge, as we Trevi tested the PRV's later and they were opening at 75 bar, however, according to our certification contractor, it was tested at 68 bar during statutory inspection. So there is a confusion...... Yes, we have a vertical discharge pipe for vent....

Is it possible that CDTP what was precalculated was wrong?? may be, they didn't consider the temp factor and back pressure accurately and so when CDTP is 68, it actually is equivalent to 75 in real environment? I'm trying to determine that correction factor..

You didn't mention the last time this valve was calibrated. Also, is 68 bar when the valve is supposed to begin to open or when it is fully open? It is possible the valve begins to open at 68, but allows pressure to increase to 75 before it is fully open.

Does this boiler have more than one PRV? Please consider the effect of time on a spring. These are not perfectly acting electronic devices, but crude safety valves to prevent a catastrophe. Most larger facilities recalibrate their PRVs on a schedule, usually 1-2 years. Based on your pressure, I assume you are a power plant. The larger plants I am familiar with have their own calibration shops.

I suggest you remove the valves(s) from the boilers and test-calibrate them regularly.

It was calibrated in 2010 and we calibrate every 2 years. Set pressure is 68 so I would assume it will fully open at 68. We have 2 valves with 68 and 70 bar set pressure and both didn't lift up, thats why im considering the CDTP calculation for these valves. It looks like a calibration issue, we missed something while calculating cold pressure.

It is common for PRVs to have a cracking (initial opening) pressure of P, but to attain full rated flow capacity at 1.1 x P ("accumulation"). 68 x 1.1 = 74.8.

Some other controls such as burner turndown or shutoff should have intervened before the PRV lifted; what were their settings, and what is the normal operating pressure? What does CDTP stand for?

Did you cold tested or online tested? After cold test this kind of set pressure differences is possible. Try to open by hand lift. If its opening by hand you have to readjust (loosen) from adjustment nut and re-trevitested. If it isn't opening by hand too. It means there is a problem on the Safety valve. Spring could be broken. Is it open bonnet or closed? Can you see the spring from outside? Another possibility could be stuck some internal components. Then it needs to be overhauled and retested.

The normal pressure is 62 bar.. We did cold and didn't consider hot online testing as we didn't need to..I can see the spring from outside, I am inclined to think about the different parameters calculation has something to do with the problem..... CDTP is cold diff test pressure which considers temp correction factor, back pressure etc

Is it opening by hand or not? When you test PRV in cold condition or on the bench at workshop you need to know CDTP how much. This information you can get from manufacturere according to type and size of PRV. It looks that you did wrong set adjustment. Readjust the PRV.

Don't assume anything, as this is a safety issue regarding a potentially explosive boiler.

• Check the last inspection reports by the Engineer/Surveyor for the company supplying burst indemnity insurance for the boiler. Make sure that all tests were witnessed by this individual. Make sure that the recommendations, if any, in the test report were carried out to the letter.
• Take the valve(s) off, and test them again; record the test(s).

The last certification report in front of me, cold test pressure was 68 bar. We cant take a shutdown because its a drastic measure but based on this experience, we may change our procedure and will implement the online testing of valves on all boilers and PV. Do you guys conduct any online or hot testing of PRV's after their cold pressure testing??

I'm going to throw my two cents in and may be wrong in every instance, however,

1. What was the process to test (certify) at 68 bar? Sure there is a specific gravity difference for water and steam. What did certifying tester utilize to test accuracy? Check with Manufacturer to confirm process is acceptable.

2. Since it is a PRV, then go ahead and perform adjustments with steam activated and in process. if, during process, your registering 50 bar, then follow manufacturer's proceduce to adjust opening of PRV at 50. Observe reaction of PRV,..slow, medium, instantantiously. Do you need to adjust further. My expereince with PRV's is that minute adjustments make dramatic changes. Proceed carefully. COnsult with manufacturer.

3. IF consistency is achieved, then proceed to perform adjustments whenever you see 55bar, 60 bar and finally, 68 bar if insured that pressure transmitter is accurate.

4. IF repeatability is not attainable, then fairly certain you need to perform corrective action against PRV.

Hope this helps and not hurts.

Thanks, we tested through conventional cold test pressure criteria i.e. test bed using Nitrogen gas. I would like to know hos often you utilize in-situ online testing after cold testing? Whats the use of cold testing if you are supposed to inspect and verify the PRV set pressure again once it's online?

My utilization of PRV's at this time is in the water utilities.

Steps 2., 3. and 4. are the application we apply. Our PRV's are diaphram operated, but, long time ago, service tech (installation and warranty) for steam sterilizers. Did not achieve pressures your experiencing but had job security with steam traps and "Pressure relief Valves.

Condensation always caused failures to both components to "activate" within its Operating points. As long as "steam traps" were consistently serviced by in house technicians and allowed condensate to be removed, "Pressure Relief Valve" always operated per spec's.

Good luck and hope this helped.

The purpose of cold testing is to raise the level of confidence that the valve will lift at that set pressure under service conditions using an inert or relatively safe fluid and that the boiler pressure will not rise to a level where the boiler bursts before the valve lifts. Steam at high pressure means that the boiler water is above its normal boiling temperature and, if the boiler fails, fragments of it will travel for considerable distance demolishing everything in their path, including humans as that water flashes to steam. Google "BLEVE" for more information.

If the cold test pressure of the PRVs is 68 bar I would expect the hot pressure to be higher due to expansion of all components including the spring increasing the spring tension, but since I am not a mechanical engineer, and do not have the design detail including the material types I could not begin to suggest how much it changes with temperature, however manufacturer's data would be a good starting point.

It is always good practise, where possible to test at operating conditions.

Do you have a spare (new, unfitted) PRV that you could set at 68 bar cold, then re-test at operating temperature without interfering with your operating system.

This should give you a good feel for what is happening. You could then even plot the PRV behaviour vs temperature and fully characterise it's performance in your configuration.

1. The thing that is preventing the valve disc from lifting is the spring (I am assuming a spring type of relief valve).

2. Therefore, it is either the valve spring did not set correctly or inadvertently adjusted after testing or some foreign materials blocking the spring (e.g. bolts, corrosion products etc.)

3. If you could not find any root cause of the problem, just retest and reset the spring pressure if required.

But there were 2 valves, none of them lifted up. May be the temp causes expansion and made spring stiff enough to increase setup pressure.

Something like this is conceivable. For instance, if the spring is stainless, it will have a greater coefficient of expansion than the iron or steel body of the valve. This might result in a somewhat greater spring force if the whole valve is heated. I doubt that this effect is enough to increase the opening pressure of the valve by 10% or so. For information of this sort, please contact the valve manufacturer.

You did not answer about other controls that should have operated before the PRV would be expected to lift. Please tell us more about that.

We just had 2 valves on steam drum. The pressure kept building up due to the failure of pressure instrument so I couldn't transmit a message to vent valve, anyway it was an instrument genuine problem but our concern is, why 2 PRV didn't come into action. You are probably right, i will start looking into Spring specs

For the third time, what were the settings of other controls that might have prevented excessive pressure rise in the first place?

yES, there is another control. Control valve will open venting system to maintain pressure but the pressure transmitter failed as well. Its being investigated by Operation so I'm not looking into it.. It was a can of worms so I'm keeping myself limited to PRV analysis :)

For the fourth time, this fails to provide useful information.

I would suggest that you resign from your job, on the simple grounds that you don't know what you are doing, and cannot provide cogent answers to simple questions.

GA. Tornodo. see other tags. He has questions on his system in numerous subjects. Seems to be reading less and leanring from CR4 Members. He does not even know how to answer some of our critical and serious suggestions.

The redundancy of testing and certifying both PRV's by the same body implies that set point for both were performed incorrectly.

Basic design construction of PRV is tension against known pressure applied to spring tensioner of PRV would hold against pressure adjusted too. Pressure greater than (with dead band) would overcome this tension applied by internal spring mechanism forcing venting to occur until such time as Pressure applied is insufficient to overcome applied tension against spring. That they are both not opening at "set pressure" of 68 bar and if I understand you correctly both certified and adjusted by one party says that this was performed incorrectly.

The question needed to ask and directed to manufacturer for this PRV is " is COLD test applied with Nitrogen gas" acceptable test method for operating set point of this PRV?

Until that is resolved, suggest that you follow manufacturers operations manual and adjust PRV against "present" steam pressure, regardless of what it presently is.

If its at 45 bar, then adjust your PRV to open at 45 bar. You need to know if the PRV is operating and if consistency is there. If so, then pretty sure that "certified testing" was missing something in the process.

That process, the acceptable "mfg." process needs to be redone and closely observed for future confidence.

In our process (water) it is factory set per our recommendation.

Upon arrival, we adjust (per mfg. procedures) when the system is online and active to a setting noted and observed on site and location. Example: we require mfg to set at factory to 68 psi. upon arrival and after installation, we observe operating conditions to be 46 psi. WE adjust the set point on site to open at 46 psi. Reset and adjust again. This to insure it works. This to insure repeatability. Once satisfied, we adjust to factory set point and monitor. adjust up or down as necessary thereafter. Since these are online continuously, we periodically test by performing the above. No certifying party involved. It is either working when adjusted too and with repeatability or removed for service and overhaul and then tested as above.

Your first step is to regain confidence in the PRV. Get your hands wet and adjust and test it. Nothing in both are industries are easy and safety insured, but, it is the industry we choose and requires us to take the steps. Plan, insure safety first and test it yourselves to satisfy yourselves.

I told you why both failed.

Second thing to check - dismantle both valves and check if the lower adjusting rings are "Totally" tightened and screwed down to bottom of each valve base.This tight assembly prevents the valve from creating a proper reaction chamber to lift the valve at set point.The adjusting rings which your maintenance may have mistakenly presumed as lock nuts designed to keep the nozzle tight to the base.

Thirdly, the valve calibration was not performed after the valve maintenance.

Was the correct medium used during testing ? it should be mentioned on your test sertificate.

If you test a safety valve with water and the set pressure is 68 bar, and than test the same valve (without adjusting anything) again, but this time with gas the opening pressure will be around 10% later, let's say 75 bar.

This mistake is verry common, but than the other way around (testing a fluid safety valve with a gas).

AFBC Boilers - Bed Coils frequent failure? I replied on the problem tto. No comments from you. What is this? a second hand boiler that you bought? and now facing with tons of problems with no available manufacturer's documents to help you? are in lahore pakistan?because a couple of boilers came but now all facing problems.

See for online test;

www.obef.com.tr

Thanks Eyuptan, I think I got what I needed

Thanks everyone

Thats all????? You are one selfish guy and opportunist guy.

What happened? I said thanks everyone for the suggestions...

Now, I will discuss all available options with my boss and then we will determine the best way to proceed further. So how comes the selfishness..

Its the first time I started analyzing PRV's and you guys helped alot to develop my knowledge. I still have lots of confusions and concerns which I will clear after meeting process guys. So I may not be able to provide all relevant info....

Again, thanks for your help, really appreciated

Thanks are nice, but they are not the same as cogent and informative answers to pertinent questions. I hope your boiler doesn't blow up, but I have no confidence in how you are managing it.

I don't know anything about this sort of thing: so general question for everyone.

How is the correction for temperature done when setting the cold differential test pressure?

Randall

if there is a temp. correction factor, this should be deliverred by manufacterer

Dear all

I see that a lot of discussion on this simple issue.

PRV adjustment should be done on line as it is always the case with Boilers on board ships with class surveyor attendance.

Keep the boiler pressure at 68 bar,Open the adjusting nut on the spring until the valve lifts.

If didn,t , you have to dismantle the valve for checking and overhauling if needed.

There is a big difference between accumalation test and PVR adjustment.

In the first you have to keep the safety valve open and burner on,and the pressure in the boiler should not increase more than 10%.

Sarwat,

rules and regulations differ from sea going vessels and shore vessels

boiler pressure gauge is giving correct reading , defective pressure gauge ,second the boiler pressure sensing line may be clogged due to poor water treatment other reason ,confirm boiler pressure gauge is giving correct pressure reading of the drum , clear the pressure sensing line

boiler safety valve is gagged (locked by bolt , two safety valve on the boiler , for testing one by one condition ,lock not removed )

vent line of the safety valve , drain clogged , water head on the safety valve which puts extra head +spring pressure relief pressure setting will change

readjust the safety valve , refer manual /maker advice /safety precaution to be taken /as dealing with high pressure and high temperature steam . safety first.

first time in forum , to be excused if suggestion off topic , good day