Constant and Variable Backpressure 4

[ChEMatt]

Having a discussion here at work and no one has a good definition of the difference and causes of constant and variable backpressure. What are the causes of constant backpressure? Likewise, what are the causes of variable backpressure?

Both links to resources/pdfs as well as straight answers are much appreciated.

Thanks for your help!!


Onwards,

Matt

 

[robsalv]

?? Huh??

What superimposed pressure conditions exist on the discharge side of a particular PSV installation? Does it vary? Is it constant? It's a case by case thing.

Sounds like you and your colleagues may be confused.


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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[Latexman]

An example of a constant backpressure is a nitrogen blanket on a flare header.

An example of a variable backpressure is the built-up backpressure due to flow (dP is proportional to Q^2).

Good luck,
Latexman

 

[JoeWong88]

An example of a constant backpressure is a flare recovery system on a flare header.

An example of a variable backpressure is pressure dumping valve maintaining process system (i.e. slugcatcher) not to exceed a set point and discharge to flare header.

JoeWong
Chemical & Process Technology

 

[ggordil]

The constant back pressure on all valves tied to a single flare is the same. So for example, the back pressure from the flare tip.

The variable back pressure has two components, built up back pressure and super-imposed back pressure.
The built-up back pressure is different for every valve and every overpressure scenario for each valve since it is dependent on the flow rate and phase of fluid to be relieved. So the same PSV will most likely have different built-up back pressures for different scenarios like external fire compared to overfilling.
The super-imposed back pressure is the pressure in the flare header due to the flow from other relief valves into the flare header for global scenarios. As an example, if there is a unit wide power failure, several vessels may relieve at the same time, this will cause the pressure in the header to go up and reduce the capacity of valves tied into that header.

 

[ChEMatt]

Well, I tried to find a third-party PSV data sheet but was unsuccessful. There are three entries dealing with backpressure on the data sheet that we do have, referred to as "constant", "variable", and "total". Of course, "total" is a sum of the constant and variable backpressures that have been calculated. What is troubling to me is that I was not able to find a good definition of what variable backpressure is, exactly. Some have pointed out that the variable backpressure might come from the fluid pushing on the backside of the disc as it is relieved. If this is the case, one would think that the manufacturer would have to supply this information.

I asked for definitions of both so that it would be clear what the differences are. We were already sure of what "constant" backpressure would be since by the very definition it is obvious, in my mind. "Variable", however, does not have a clear source.

From the responses above I believe Latexman and ggordil have provided the best definitions.

One final request, does anyone have a link to a PSV data sheet? I did not develop our internal one and I would like to compare what we have to others.

Thanks to everyone for their assistance.

Onwards,

Matt

 

[insult2injury]

Take a look at the API standard 520.

I2I

 

[robsalv]

I2I beat me to it!


On our spec sheets we have "Superimposed", "Developed" and "Total".

Total is an important figure for determining when a bellows type becomes the preferred. If total => 10% of set, then we go to a bellows type SV.

"Developed" is exactly that, the back pressure developed in the discharge at 10% accumulation when relieving at the design contingency conditions.



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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[ChEMatt]

I think I like the terms that robsalv mentions. I may change our data sheet to reflect that.

Thanks to all who replied!

Onwards,

Matt

 

[JoeWong88]

When i first seen datasheet stating "constant", "variable", and "total" is a sum of the constant and variable backpressures that have been calculated., i failed to understand them...

This is a set definition are commonly used by Instrument and PSV vendor which have created thousand questions and debates.

"constant" back pressure ? Can we really have a constant backpressure to PSV ? Say a PCV connected to flare header dumping excess fluid in system. The flow may varies from time to time...the back pressure to PSVs will change from time to time ? Can we have a "constant" backpressure ?

"variable" back pressure ? Is it related to back pressure cause by other device like above mentioned case ? Or solely pressure drop due to the relieve of the particular PSV ? Or pressure drop due to the PSV relieve flow plus the impact by other PSV which potentially relief simultaneously ?

"total" is a sum of the constant and variable backpressures that have been calculated. If both constant and variable are unclear, what about the "total" ?

Anyway, above have been used by Instrument and vendor for whatsoever reason. We as process engineer would really need to understand how their interprets them and put the "suitable" figures into the column.


Let see what the definitions by API 521 which i believe most relevant and understandable :

3.5
back pressure
pressure that exists at the outlet of a pressure-relief device as a result of the pressure in the discharge system
NOTE The back pressure is the sum of the superimposed and built-up back pressures.

3.11
built-up back pressure
increase in pressure at the outlet of a pressure-relief device that develops as a result of flow after the pressurerelief
device opens

3.75
superimposed back pressure
static pressure that exists at the outlet of a pressure-relief device at the time the device is required to operate
NOTE It is the result of pressure in the discharge system coming from other sources and can be constant or variable.


"Superimposed", "Developed" and "Total" by robsalv is pretty close to above.

JoeWong
Chemical & Process Technology

 

[AB6005]

Contatnt back pressure is northing but the pressure at the PSV tail pipe when it is in closed condition.


Superimposed back pressure is northing but but the pressure at the PSV tail pipe when it is relieving.

 

[ChEMatt]

AB6005,

How then is the superimposed back pressure calculated? When the PSV is relieving, what is the pressure at the outlet? Can you give me an example?

I appreciate the help!!

Onwards,

Matt

 

[skearse]

Superimposed backpressure is going to be the static pressure in the discharge line at the time the valve is required to open. For example, if you are relieving into a closed vessel that maintains a 2 psig nitrogen blanket at all times, then your superimposed back pressure will be 2 psig. For an atmospheric discharge it would be 0 psig.

Built up backpressure is the pressure drop caused by the flow from the relieving fluid. It's calculated by using Crane's to determine the dP in the line, based on the PSV RATED VALVE capacity flow (per API) unless it's a modulating valve, in which case you are permitted to use the actual relief flow.

 

[sheiko]

Let's get practical by one example:

- set pressure = 10 barg

- superimposed backpressure (min) = 0.3 barg
- superimposed backpressure (max) = 1.5 barg

- built-up backpressure = 2.5 bar

- constant backpressure = constant portion of superimposed backpressure = 0.3 barg

- variable backpressure = variable portion of superimposed backpressure + built-up backpressure = 1.2 + 2.5 = 3.7 barg

- total backpressure = 1.5 + 2.5 = 0.3 + 3.7 = 4 barg

Any objection?


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

 

[JoeWong88]

Sheiko,
I don't disagree with what you have done, however sometime proper communication is crucial.

Set pressure
Some could interpret set pressure as pressure when PSV start to open and other interpret it as Pressure for Net Spring Setting (as what we have discussed via personal communications.)...This is one potential definition and communication issue.


superimposed backpressure (min and max)
Maximum superimposed backpressure could be derived from scenario A i.e. Total power failure. However, if one of your PSV relieving scenario (control case) is single relief scenario i.e block discharge but not in total power failure...This is another potential definition and communication issue.

constant backpressure
Process engineer may interpret it as minimum superimposed backpressure as he/she think that minimum pressure in header lead to maximum Mach no or whatsoever reason...

Another process engineer may interpret it as maximum superimposed backpressure as he/she think vendor will use this to derive the spring setting. Maximum pressure will be used to avoid PSV opening at pressure higher than set pressure .

This is another potential definition and communication issue.

Variable backpressure
One engineer may interpret as maximum superimposed backpressure. Why ? He/she imagine that the flare system normally see minimum superimposed backpressure, when one of the PCV open discharge to flare system and lead to maximum superimposed backpressure, the flow from this PCV may varies. So the variable backpressure is the maximum superimposed backpressure.

Another engineer would interpret it as minimum superimposed backpressure plus built-up backpressure. He/She consider the maximum superimposed is intermittent. It is not present when PSV is open.

Another engineer would say NO. It should be the Backpressure (=builtup + maximum superimposed). This figure will be used by vendor to select a correct PSV type (conventional vs balance).


This is another potential definition and communication issue.


I have been working on many projects with different clients at international and state level. Similar confusion continue and continue... Again, my advice is make the definition clear in the PHILOSOPHY and SPECIFICATION.


JoeWong
Chemical & Process Technology

 

[robsalv]

From API520 part 1.

3.3 backpressure
The pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system.
Backpressure is the sum of the superimposed and built-up backpressures.


3.7 built-up backpressure
The increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure
relief device opens.


3.31 opening pressure
The value of increasing inlet static pressure at which there is a measurable lift of the disc or at which discharge of the fluid becomes continuous, as determined by seeing, feeling, or hearing.


3.49 set pressure
The inlet gauge pressure at which the pressure relief device is set to open under service conditions.


3.53 superimposed backpressure
The static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate.
Superimposed backpressure is the result of pressure in the discharge system coming from other sources and may be
constant or variable.


I'm not sure what the confusion is?

- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[sheiko]

robsalv,

Do you agree with my example (namely, that constant backpressure is indeed the minimum superimposed backpressure)?

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