Constantly Relieving RVs, Who Cares?? 11

[kykuran]

I get the following question a lot:

"What difference does it make if a relief valve(of any type) constantly relieves or not? If the system overpressures, then it will still relieve."

The first time someone asked me this, I was shocked. I've been telling them the typical problems:

-Could cut the seats and might not reseat.
-Premature valve failure.
-Can't guarantee that the valve will function properly.

None of these replies satisfies them. They seem to think that it is just to satisfy Code, OSHA, etc (which to them are all nonsense and just makes them spend more money.)

Does anyone have any better practical responses? Not just "because code says so."

Thanks

 

[Guidoo]

Kykuran,

Apart from the arguments you already gave, I would mention the fact that a relief valve is the last line of defense against catastrophic failure. Some people may think that relief valves always work when you need them, but unfortunately this is not true. According to CCPS Generic Failure Rate Database, mean Probability of Failure to Open on Demand is about 0.0002 for a spring-loaded relief valve, and about 0.0042 for a pilot operated relief valve. In order to make calculations easy, let's use a Probability of Failure on Demand (PFD) of 0.001.

Normally, you would expect that the demand on a relief valve is say once every ten years. A demand occurs for example when the Process Control System fails and operator fails to take corrective action. In this case, frequency of a catastophic failure would be once every 10/0.001= 10000 years. This falls in the range of what is usually called tolerable.

In "your" case, the demand on the relief valve is say ten times per year. Frequency of a catastrophic failure now becomes 0.1/0.001=100 years. This does not look acceptable. It would mean that if you have ten such relief valves, it is quite likely that you have a catastrophic failure within 10 years from now...

So to make a long story short, when your relief valves are called upon frequently, chances of a catastrophy (maybe resulting in fatalities!) are unacceptably high.

 

[Guidoo]

You can say it even shorter:

If you have such frequent demands on your last line of defense, you're only one failure away of a catastrophy...

 

[kykuran]

Guidoo,

Nice responses!

I am not a statistician, but understand the logic. Unfortunately, the demand on these RVs is greater than you have assumed, although not wide open.

The problem is the differential between set pressure and operating is unacceptable. The (perceived) costs to correct the problem are greater than the concerns.

I hope I am not alone with having to deal with this attitude. Actually, I hope I am for the safety of others.

Thanks

 

[bchoate]

bchoate
Aside from mechanical reasons for not having a relief valve that reliefs constantly, there are a few other considerations. Why is operation such that the RV has to relieve constantly. It seems that operating conditions and normal extremes are too close to the vessel's rating and consequently the RV's setting. I see a fundamental design issue with the vessel used for the process if the RV has to relief constantly. The function of the RV is to protect the vessel or equipment in case of some unforeseen, extreme, perhaps catastrophic operating conditions as one post mentioned. The vessel rating should be such that it encompasses, with a safety margin, the extremes of the normal operation to be conducted in it. The RV is sized according to the vessel rating and the materials to be contained in the vessel.

A second consideration is, "where is this valve relieving to and what is relieving?" Dangerous materials discharged to the atmosphere are a problem with both personnel safety and with regulatory compliance. If the valve is discharged into a vent header which connects to some abatement device such as a flare or a thermal oxidizer, one has to consider the physical and economic impacts on those systems as well.

Long story short, a constantly relieving RV is never a desirable scenario and suggests indifference to a variety of issues which professional engineers and others should be attentive to.
Bill Choate

 

[pmureiko]

Is the relief valve (RV) constantly or frequently relieving? A lot of expensive product is being lost if the RV is constantly venting. If the valve does not reseat, then you will have constant leakage operators may not be aware of.

The unit may be operating at a net loss. The cost of product lost through the RV may be more than the value obtained from operating close to the vessel's pressure rating. The cost of product may also involve downstream handling costs like extra steam at the flare. Management is not considering the cost of these things, since they don't think it is happening.

Don't just accept an answer that venting is not constant or is infrequent. Check if the RV discharge line is running hot or cold? An operator may not care or realize the consequences of running this way.

 

[saxon]

kykuran, Constant relieving PSV's are an indication that the system is operating beyond its MAWP, or that prior calibraton and maintenance activities have been passed over in the interest of whatever, or that the system was improperly designed in the first place. A problem? Yes. Risky business? Yes. An accident waiting to happen? Yes. I have one question, Who's got the resposibility to notify the families that Daddy is never coming home again because he died in an explosion at the plant?

Now do you see the importance?
saxon

 

[kykuran]

Please don't mistake my original post. I whole heartedly agree that this is a problem.

I also agree that this is not a professional attitude at all. I believe the people asking me this should be embarrased.

When you are pounded on a regular basis with questions like these, it eventually makes you wonder if you could be over reacting (just barely though). It appears that I don't have much support either. Fortunately, I haven't budged.

 

[athomas236]

I always look on PSV's as the last line of defence when all goes wrong. They are not intended to overcome faulty design.

athomas236

 

[MortenA]

Also consider this:

If the valves lft how do you know they do not exceed 10% overpressur?

The plat was not designed to have RVs that lift i bet?

So how can you be sure that the 10% limit is not exceeded too?

Best regards

Morten

 

[sshep]

An easier answer:

In the USA there are many processes covered by OSHA 1910.119 (and other regs) for which safe upper operating limits must be established. It is practically by definition that these are below the associated relief settings. Establishing a pattern of routine operations which go above these limits could be interpreted as a willful violation of the code, and can result in criminal prosecution of your plant management.

 

[MJCronin]

One question....

Aside from the excellent, well thought out answers expressed above.....

Is there some way that you can get these management people making these statements to stand NEXT TO THE VALVES while they relieve ?

The best valves, of course, are the steam relief valves discharging to atmosphere..(like on an HP steam line)...have these geniuses stand really, really close. Tell them how anyone "worth his salt" should investigate these frequent valve actuations.....Yeah, ....really, really close...

An old friend of mine cannot use one ear because he was inspecting an instrument when a nearby steam valve actuated.

Gimmie a star......

MJC

 

[aspearin1]

One purple star... You got it!

Aaron Spearin, EIT
ChemE, M.E.
"The only constant in life is change." -Dan Andia; 1999, Chemical Engineering Progress

 

[kykuran]

I wanted to answer a few of the questions above.

[highlight]1) Why is operation such that the RV has to relieve constantly?[/highlight]
I've been told that neither the system design nor the operating conditions have changed. Everyone is claiming ignorance. I am new, so either I only have what is given to me or what I can find.

[highlight]2) Where is this valve relieving to and what is relieving?[/highlight]
The valve relieves to atmosphere and the services are high and low pressure steam.

[highlight]3) Is the relief valve (RV) constantly or frequently relieving?[/highlight]
Both. At times they are constant. Overall, the demand is frequent.

[highlight]4) If the valves lift how do you know they do not exceed 10% overpressure?[/highlight]
There are pressure and temperature transmitters that are stored in an accessible database. The system is called PI process book. It is very useful. In most cases, there are multiple valves; therefore 16% overpressure is allowed.

[highlight]5) Is there some way that you can get these management people making these statements to stand NEXT TO THE VALVES while they relieve?[/highlight]
NO! Most of these areas already require double hearing protection anyway. Neither do I make it a habit to climb around the plant with management nor do they with me. I sometimes wonder if they know how to climb a ladder.

 

[kykuran]

Along the same subject...

Many of these valves have Car-Sealed Isolation Valves. The Car-Seals are tracked weekly and monthly religiously. However, these same people ask the following question a lot as well:

"Who cares if the Stem Packing is leaking? We just won't be able to isolate the RV."

Again, another stupid question that is asked repeatedly. I tell them that if the RV needs to be repaired, inspected, etc. You might have to shut the whole system down to do so. Their response is, "That won't ever happen."

This mentality only makes the job tougher. It's like trying to explain to children why it's necessary to pick up their toys.

 

[jay165]

Not sure about your state, but here in La. it is against the law to put block valves (even car sealed or locked open)under relief valves for steam boiler service. We used to get around a lot of the requirements by calling steam heat exchangers "process coolers", one example being the FCCU effluent heat exchangers. If it was an actual fired boiler however, the rule applied. I would check your state's boiler laws.

In addition, especially with fuel prices the way they are are, all that steam has got to be wasting $$. I remember back in the '80s we audited the refinery where I worked for steam leaks and it was really a shocker. The payback for repairing the leaks (including some expensive valve replacements) was in weeks. We were actually able to eliminate outside fuel gas purchases.

 

[MortenA]

Kukuran

With reference to an earlier posting from you:

4) multiple valves are only allowed if they are online (i seem to remember that they also have to be of different sizes - but im not sure)

Best regards

Morten

 

[krossview]

Kukuran,

One way you really will convince management that they have problem, when you provide them with hard numbers. Express in $$$$ in lost fuel gas cost on an annual basis (work with large $$$) to generate the steam lost thru the relief valves.

To do this estimate how much each PSV reliefs based on measured operating conditions from you database(s). If you really want to know what a PSV does, have it bench test at the exact conditions in the plant and measure the relief flow.

Hope this give a different point of view on it

Krossview/OK

 

[MedicineEng]

Kykuran:
When was the last time that someone checked those valves?
We are here discussing design and operation, when the problem can be quite simply, lack of maintenance ( spring loose, seat damaged, etc.)
When I first arrive to my site, I realized that nobody cared about safety valves and that was a "grey" area.
When I first asked "who is in charge of the checking and calibration of the safety valves?", I saw a lot of nervous coughing and whistling to the side.
By that time, we initiated a maintenance and calibration schedule for the safety valves where yearly we clean and check if the safety valve is opening at the pressure that it should.
Regarding the issue with your bosses, you can make a very simple calculation on the steam that you are loosing:
I assume that you have a condensate line/tank to the recover the condensates. As such, your steam system is a closed loop and in ideal situation, you should not spend any water to feed the boiler. You put a watermeter in your boiler room to see the water consumption. Then, you assume that for instance 75% ( the other 25% are for other leaks in your steam pipeline. Of course you set this percentage according with your reality) of that consumption is due to the safety valves relief. Then, you calculate the ammount of diesel, gas or fuel that you spend to heat up that water from ambient temperature to the steam temperature (take into consideration the efficiency of your boiler). Wrap up all that in nice graphics and payback times and you will see that they will understand.

Hope that helps.

PR

 

[johnnygeneric]

I agree with jay150. Put it in a form the management will understand. Do the math on the wasted energy costs of letting the steam vent. That's money right out the window.

When management sees the costs they'll shout "Holy Cow! We've GOT to fix this!" That way management can say "look, we saved all this money AND," with a big grin, "we made the plant safer!"

Gary Futral