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CV Failure Philosophy
4

CV Failure Philosophy

CV Failure Philosophy

(OP)
Let's say there is a control valve with a system upstream designed for 200 psig (i.e. the upstream relief valve set pressure).  Operations plans to actually run the upstream equipment at no more than 150 psig.

There's also a a relief valve downstream of the control valve (system downstream design = 50 psig).  When sizing this relief valve for a CV fail open case, what does one assume for the CV upstream pressure in these calcs?  Would you use:

a.) 150 psig
b.) 200 psig

Assume the client is indecisive and leaving it up to the designer.

RE: CV Failure Philosophy

Personally, I would size the downstream PSV for whichever upstream pressure gives rise to the higher mass flow through (and thus downstream of) the failed control valve.

Regards,

SNORGY.

RE: CV Failure Philosophy

For the information given, I would use 90% of the upstream safety valve setting i.e. 180-psig.
It is not likely for the control valve to fail at the same time the safety valve is 200-psig.
But the upstream is capable to continuously operate at 180-psig.
Planning to operate at no more than 150-psig is only good intensions.

RE: CV Failure Philosophy

(OP)
Thanks, Chance- my argument, exactly.  There is quite a bit of debate between myself and several another engineers on this.

The designer has no idea how operations will run the plant after startup and my experience with operations companies is that they're always pushing the limits of the equipment in terms of flow and pressure.  

If anyone else has an opinion on this (latexman?), I'd like to hear it.

RE: CV Failure Philosophy

Will it matter?

Since PSV's only comes in fixed sizes - being really cleaver chosing your relief rates dosnt always make a difference?

Why dont you check for both pressures - and if they both ead to the same size go for the larger?

Best regards

Morten

RE: CV Failure Philosophy

(OP)
Doing the math, it can matter.  Quite a bit.  In some situations this assumption will cause the valve to go up 1-2 sizes.

RE: CV Failure Philosophy

2
Who is to say that if the upstream side went into relief an operator wouldn't try to depressure that side by manually opening the control valve all the way?

There is no engineering merit in just choosing an arbitrary upstream pressure like 180 psig. If anything, I can justify saying the upstream operating pressure would be 175 psig because it is pretty common to set the design pressure the greater of 10% or 25 psig higher than the maximum operating pressure.

You must size the downstream PSV using a delta P of 150 psi (200 - 50) and the full open Cv of the control valve.

RE: CV Failure Philosophy

Jefka - it cant take that long to size in both cases - but then i will assume that you have done just that and found that its larger in the HP case - so i will rest my.

Best regards

Morten

RE: CV Failure Philosophy

i can't really argue with Chance's suggestion since above 90% of set, you are likely approaching "weeping" for that PSV and one should not choose to operate there.  

however, another consideration would be to size for the MAWP of the upstream equipment which might be more than the 200 psig setting of the PSV.

Operations is notorius for changing PSV settings to match the MAWP and not look at all the downstream ramifications.

also consider a slightly smaller control valve or reduced trim if practical.  this could limit the flow and help out.

RE: CV Failure Philosophy

Jefka, I agree with Chance17.

API 521 is very clear that we only need to look at one unrelated event at a time.

For the "Control valve fail open case" the upstream pressure can be taken as the normal operating pressure, except if you can see a reason why it would increase when the control valve fails open.

That being said, the normal operating pressure is typically 10% or 25 psi below the set pressure.

Also remember that for a control valve that fails open, the governing case may be after you lose liquid level in the upstream vessel.

Bottom line - in your case I would use 175 psig inlet pressure.

If they want to you to use 150 psig, then they need to change the relief valve set pressure to 175 psig.

Cilliers
 

RE: CV Failure Philosophy

You keep on talking about a failed-open control valve and this is indeed a credible scenario but I would like someone to justify that an operator cannot rationalize to manually open that control valve to depressure the upstream system going into relief?

RE: CV Failure Philosophy

All
Just follow Pleckner piece of advice! It's worth it.
Some people would even use the upstream relieving pressure, ie 220 psig...  

"We don't believe things because they are true, things are true because we believe them."

RE: CV Failure Philosophy

Fully agree with Pleckner - use the 200 Psig case.

RE: CV Failure Philosophy

Yes, Sheiko is even more correct. Upstream pressure should be 220 psig (unless relieving scenario is fire of course)... I wasn't thinking it all the way through; need to include the allowable overpressure during relief.

RE: CV Failure Philosophy

Not mentionning that, if there is a manual bypass valve (for maintenance, for startup,...) in parallel to the main control valve we're talking about, the rated Cv of the bypass valve should be added to the rated Cv of the main control valve for determining the relieving flowrate...

"We don't believe things because they are true, things are true because we believe them."

RE: CV Failure Philosophy

pleckner,

- So your scenario is: Vessel inlet control valve fails open. Overpressure the vessel. Operator or instrumentation opens the control valve in question.

If this is a valid scenario, then I agree that the control valve inlet pressure should be based on accummulated pres.

- For the control valve fail open scenario however, the inlet pressure should be based on the normal operating pressure.

In both cases you can take credit for the flow that continues to flow downstream.

Cilliers

RE: CV Failure Philosophy

CJKruger: What do you define as normal operating pressure?

RE: CV Failure Philosophy

As stated by the first post, I would take Normal Operating to be 150 psi

Note also that differential pressure of the control valve in this case is the normal operating pressure at the inlet and the relief pressure plus over pressure of lower system at the outlet.
so, assuming 10% overpressure, set is at 50 psi... gives us 55 psi.  Total relieving flow for valve fail open scenario to consider 150-55=95 psi delta across the valve.

so, one needs to calculate the flow through the valve with normal inlet conditions with a pressure drop of 95 psi

RE: CV Failure Philosophy

I'd like to know the function of the CV?  Control pressure?  Upstream or downstream pressure?  Control flow?  Liquid or gas?  Something else?  The answers to these will help determine what is going to happen when in "automatic".  As others have stated, what the operations department allows during emergency conditions will determine what sizing secenarios to consider credible.  If the plant does not have an operations department yet (design phase), then the assumptions made will determine and/or limit them on what they can do safely.

Good luck,
Latexman

RE: CV Failure Philosophy

(OP)
@Latexman: I was speaking for a completely hypothetical but let's say the control valve is an FCV taking plant air from an accumulator to a downstream user.  That's a case where operations may be tempted to increase the pressure to increase the holding capacity of the vessel.

Manually relieving the upstream equipment is a very good point.  However, I suppose am primarily concerned with a more general question:

Is assuming that operations may not always run the equipment as in the operations manual considered a "double jeopardy" case?  While most operators would know not to mess with the PSV's or CV's, I've known some who would change an operating pressure or flowrate without hesitation.

RE: CV Failure Philosophy

from an operations viewpoint, any level set-point between high and low level alarms or any pressure set-point between alarms or essentially from vacuum relief and the PSV setting is typically considered to be fair game.

considering the wide range of conditions found during start-up, shut-down, responding to upsets, striving for efficiencies, etc.  you essentially have to assume the operators will run it high, wide and handsome.

RE: CV Failure Philosophy


What I would use is (200*1.1)-(50*1.1) = 165 at 100% open Cv for the mass rate capacity for the 50-lb PSV.
 

RE: CV Failure Philosophy

This post has already had replies from many of us. I hope I am not that late in putting my views. Here is what I would do:

I would base my relief rate on maximum operating pressure in upstream vessel i.e. 150 psig in above case. My argument is two-fold:

1) API 521, 2007 edition, section 5.10.3 asks designer to consider operating pressure upstream and relieving pressure downstream for relief calculations for control valve failure

2) I don't buy the argument that operations people try to utilise margin between operating and design to increase throughput/profit margin etc. It is not job of designer to consider these cases. Operations people have to ensure that existing PSV is sufficient for their new operating case.

In example given above, should designer consider the possibility that in future operations department may realise that upstream vessel MAWP is 250 psig and may start operating close to it?

Codes take care of this with margins. Being strict on upstream side and ignorant on downstream side is not how I would design the system.

Regards,

Sachin

RE: CV Failure Philosophy

chemsac2:

I couldn't disagree with you more. Operations is not responsible for ensuring the PSV is designed correctly for any condition. That is the responsibility of the Process Design Engineer or the unit Process Engineer. Operations cannot just change the operating parameters without checking with engineering if it is safe to do so.

Yes, to boost profits, operations will take whatever they can get, even if it means boosting pressure up to the MAWP of a vessel unless they are stopped by someone or procedure.

Psafety has got it 100% correct.

RE: CV Failure Philosophy

Pleckner,

I do not have operations background and hence may be little ignorant about operator behaviour. Consider this post as my way of getting first hand information on operator behaviour.

However, as a designer I would argue against using 200 psig upstream pressure on two counts: technical and administrative.

Technical arguments:

1) When I assume upstream system at design pressure, I assume pressure control system in upstream vessel is not working. Even if one does not consider favourable response of instrumentation system, it is highly unlikely that upstream vessel would reach design pressure due to something that happens downstream.

2) It depends on upstream vessel make-up stream. If make-up is much less than outflow, pressure in upstream vessel would actually diminish.

3) Consider make-up is significant. Now gas blow-by occurs with upstream vessel reaching design pressure. Due to increased pressure in upstream vessel, make-up would decrease (you should not consider negative response of make-up control system).

4) It depends on relative volumes of upstream and downstream vessels as well. In one particular case of ours, low make-up stream flowrate and large volume of low pressure system almost eliminated gas blow-by case.

5) Assume, I have sized PSV considering 150 psig in upstream. Now assume control valve fails during operation and upstream vessel somehow reaches design pressure. It still may not be an issue as following margins may help:
- additional flow is taken care of by rated flow through orifice
- Pressure in downstream vessel may exceed design pressure, but since MAWP is generally higher than design pressure no problem occurs
- Combined flow of PSV and downstream pressure control system (even without favourable response) at relieving pressure may avert the situation

6) Generally PSV for gas blow-by case is calculated using bypass valve full open. When gas blow-by occurs, bypass may not be open providing sufficient margin. Here I assume, administrative controls prohibit operators from keeping bypass open in their bid to increase throughput.

Administrative arguments:

During my limited interactions with site people I found them to be serious about PSV operation. If they think something can lead to PSV blowing, I have seen them avoiding it. Also, system of permits to be obtained from technical departments would also help avoid intentional maloperation.

Summary:

I would consider design pressure of upstream vessel (200 psig) for gas blow-by case only I see a chance of upstream vessel reaching design pressure on control valve failure. E.g. upstream vessel level is controlled by a level control valve in its feed.

I would explore such possibilities during HAZOP

Regards,

Sachin

RE: CV Failure Philosophy

think of operators at the plant as drivers on the road.  some will drive the speed limit (i.e., operate at design pressure) and others will drive as fast as they think they can get away with and not get a ticket (i.e., operate closer to the MAWP or PSV set pressure).

for a relatively high pressure plant (3000 psig), we upgraded to pilot operated valves so we could run closer to the set pressure.  and one operations leader told the PSV company to not bother delivering any PSV's that were on the low side of the tolerance.  

a +/- 3% is +/- 90 psig and we wanted to run at 2930 psig.  i have also heard tales of PSV's being sent back if they were not on +2-3% side.

RE: CV Failure Philosophy

(OP)
@BenThayer:  That's a very good point.

Let's say you size the above PSV example for an upstream operational pressure of 150 psig.

Picture yourself walking into this plant and you see a pressure gauge on the equipment reading 155 psig.  How comfortable are you standing there?  Maybe the gauge reads 160 psig.  Now how comfortable are you?

RE: CV Failure Philosophy

In addition to the above questions, a large part of our work involves going into aging facilities, re-rating them, pushing the limits of flows, pressures, and temperatures.  I don't want to leave a facility in the hands of an operator (or a future owner) that will arbitrarily jack the pressure up to 199 Psig with a 200 Psig limit.  Having said that, I've heard of instructions to operations teams "push it till the valve simmers and then back off a bit".

 

RE: CV Failure Philosophy

After I posted my reply, I thought myself as an operator. I was expecting operator to be a rational being. But then I know how we use non-rational ways like judgement, rules-of-thumb due to schedule pressure during design stage.

In the end I thought it would be better to be safe than sorry. However, my worry is that assuming operator to be non-rational means thinking of lot many possibilities which I would not consider normally. Probably that is the difference between theory and practice.

Anyway thanks BenThayer, Jefka and Pleckner.

Regards,

Sachin

RE: CV Failure Philosophy

So be CONSERVATIVE!

"We don't believe things because they are true, things are true because we believe them."

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