4 replies to this topic

[zzackson]

Dears,

I'm a junior project engineer in a chemical processing plant,

and wonder what is governing case of the psv installed in the resin conveying air blower system outlet, 

and how I could calculated the required capacity. Here's the case.

 

1. PSV information

- Air blower has a psv at air blower outlet with set pressure of "0.9kg/cm2g".

- The PSV has no datasheet in my company except for the name plate in site with the calculated capacity of "1500m3/hr"

- the unit installed at 1990's, no record of the psv case.

2. Logic

- The blower also has a shut-off logic with outlet high pressure of "0.65kg/cm2g".

3. Blower Condition

- Air blowing capacity : 5700m3/hr

- Outlet Pressure : 0.35kg/cm2g

- Temperature : 50deg.C

4. Problem

- If this psv logic is for "block out case", then the psv has to be changed with "5700m3/hr capacity" other than "1500m3/hr".

5. Expected protect target equipment

- The expected protect target equipment of the psv is "filter with design pressure 1.03 kg/cm2g". Line class is upper spec than psv set pressure

6. Expected Governing Case

- There's no external fire source around 15m diameter,

  also the blocked out case is already protected with shut-down logic, which is mentioned at "API521, 5.5"

 "the system being protected should also be considered in determining the required relieving rate."

- So, I have expected the case of the psv as the protect of the filter from "Air blower outlet pressure fluctuation",

  I am regarding the psv doesn't need to be installed at the air blower outlet if the shutdown logic is set under the equipment design pressure and a relief valve is just enough.

 

7. Question

1) I would like to ask why this PSV installed with what safety logic around the API CODE.

2) If this is just a case with my expection : pressure fluctuation, how can i define the required capacity? 

3) is this psv need to be installed really?

 

Attachment : P&ID jpeg

 

Thanks

Attached Files

•  sample p&id.pdf   78.94KB   45 downloads

Edited by zzackson, 06 September 2016 - 06:44 PM.

[Saml]

 

4. Problem

- If this psv logic is for "block out case", then the psv has to be changed with "5700m3/hr capacity" other than "1500m3/hr".

5. Expected protect target equipment

- The expected protect target equipment of the psv is "filter with design pressure 1.03 kg/cm2g". Line class is upper spec than psv set pressure

 

 

 

API 521: "The quantity of material to be relieved should be determined at conditions that correspond to relieving

conditions instead of at normal operating conditions. The required relieving rate is often reduced appreciably

when this difference in conditions is considered"

So it is not the flow at 0,35 kg/cm2g what you should considered, It should be the compressor flow at 0.99 kg/cm2g (10% accumulation)

 

The protected equipment is probably the same compressor. Not for mechanical failure of the casing, but for overheating. This valve probably provides a minimum flow if the outlet is blocked and the switch fails (or is disabled). Perhaps you can find more details about this in the compressor manual.

 

Regards

[zzackson]

Dear Saml,

 

Thank you for your kind review and guide,

But, I cannot regard the pressure will charge up to PSV set pressure because of "high pressure Shut-off logic" at the blower discharge line. it instructs "0.65kg/cm2g".

So, if the blower discharge pressure accumulate up to 0.65kg/cm2g, the air blower will be shut-off other than psv set pressure 0.99kg/cm2g.

Also this is guided as to API521 5.5 "the system being protected should also be considered in determining the required relieving rate"

 

It is very easy when psv required capacity  set up as a normal condition's or based on performance curve at 0.99kg/cm2g,

but I do not want to spend the cost to install new psv of nor. condition capa but also do not have the performance curve.

(Same Issues like this package are almost 10 set.)  

 

 I want to set up the installed psv concept as to "blower discharge pressure fluctuation", and want to know how to set up the "required Capacity" matching  "Blower discharge pressure fluctuation" case if vendor data is absent, also don't want to change three times larger psv than existing.

 

So I want to be guided my expection case is right or not, hot to set up "required capacity" at my case.

 

 

Thanks,

Edited by zzackson, 08 September 2016 - 02:50 AM.

[Saml]

You are the engineer doing this evaluation.

 

 

First:

Under 15 psig pressure it does not fall under ASME code. You can use the concepts, but you are not mandated to go by it.

API 521 is designed primary for pressure equipment in refineries. Again, you can use the concepts.

Also, I suspect that the valve is for protection of the compressor internals, not mechanical failure of the pressure casing (unless the item you are protecting is the filter). In this case, you should refer to the requirements of the construction code for the filter, or use a sound engineering approach to determine the maximum overpressure you will admit.

 

 

Second:

Now I assume that YOU decided to use API 521 as a design basis(I emphasise the you, because as the engineer doing the design is it your responsibility to decide this). In this case:

If you are not sizing the PSV for full relief under blocked outlet because of the switch, then you are actually saying that your overpressure protection is the Switch.  API requires a certain level of reliability for this. Usually this is implemented in a Safety Instrumented System. Probably not what you have. 

API is clear "favorable response of conventional instrumentation should not be assumed when sizing individual process

equipment pressure relief"

 

 

Third: 

"do not have the performance curve"

You cannot design without data. I you need to obtain the performance curve, that is not difficult. It is a performance test.

 

Fourth:

API is a guideline of common scenarios. You are free (and also obliged to) to consider other possible ones. So if you consider a "blower discharge pressure fluctuation" scenario (whatever it is) as a credible one and the limiting one. Then document it and design for it.

 

 

Fifth

"I do not want to spend the cost to install new psv"

That is not an engineering problem. It is a problem of being a responsible operator. You are not mandated to know there is a problem with the design. But once you know, you have to use good engineering to solve it. And in fact, if you suspect that there is a risk to employees from this equipment not meeting a good design criteria, you should recommend stopping it until the problem is solved.

However: I suspect the risk here is galling the compressor, not loss of containment, or explosion. So you are free to suggest acceptance of that risk. Document that this is the case if you reach that conclusion.

[zzackson]

Dear Saml,

Thank you for more issues with your logic stages.

1st & 2nd :

As you said, the equipment at compressor discharge is not important, because my company specification is higher than api, astm, asme code, all equipment has to be designed with min mawp over 3.5 kg/cm2g even if the vessel work on atm. For the compressor, it is not just a closed loop compressor; it is opened and connected with atmosphere.

Also, we've got the answer from air blower vendor that there's no limit "shut-off pressure" of their blower, no impact on casing for the high pressure by blower.

At the guide by API 521 4.2.4, it instruct “instrumentation should not be a substitute for properly sized psv" against single -jeopardy overpressure scenarios, also this section instructs

"if the psv is impractical and reliance on instrumented safeguards is needed, then do it by circumstances".

So, yet, the protect target & usage of API Code guide is opened choices on my mind.

 

Third : It is not the point because I'm thinking "psv is impractical" because a series of protect instrument is set-up and he is protecting the pressure high risk, though I'm agree your opinion if I can order to operator to shut-off the process and check the performance.

 

Fourth : one hundred percent agree.

 

Fifth : About the work logic, I always think the safety first and the cost for second criteria.

And regarding there's no need to be installed of psv at "Air blower discharge line, if the instrumentation set-up"

This is a tiny example. Other bottleneck problem during operating over design margin is a really critical problem as you said.

 

 

I will follow-up your 4th guide on this problem.

Thanks,

Edited by zzackson, 12 September 2016 - 01:42 AM.