16 replies to this topic

[pravin164]

Hi,

I need help on the following issue.

1. I am refering the API-520 (part 1 & part 2) since last three years. i have rated and design the PSVs mostly for critical flow behavior following the method given in para 3.6.1 in API-520 part-1.the sizing philosophy in this method uses the capacity correction factor (Kb) to compensate for back pressure (Normally for balanced bellow) for conventional and pilot operated valve Kb =1.

2. Parallaly there is method also for subcritical flow behavior para 3.6.3 in API-520 part-1, which can be used for the relief system having more backpressure.

3. There is one alternate method para 3.6.4 in API-520 part-1 for PSv sizing for subcritical flow which allows you to use Kb values for convnetional and pilot operated PSVs. This is originally the critical flow sizing method.

Now the query is when and why this Kb factor is used for conventional and pilot operated psvs remebering the fact that pilot operated valve is used specially with the system having high back pressure, which normally results in subcritical flow regime at outlet.

[fallah]

pravin,

At first, to prevent any confusing when you refer to a code/standard you are asked to specify the Edition No.

The method included in 3.6.4 really is terminated to the method in 3.6.3 in sizing result point of view, because K_b in 3.6.4 is derived by putting the subcritical flow equation equal to the critical flow equation. In fact this is a tricky method to use familiar critical flow equation instead subcritical flow equation.

Hope, now it is clear for you...

Fallah

[pravin164]

Sir,

My query is when and why conventional PSV is required for subcritical flow behavior (i presume normally happens in system with high backpressure)

pravin

[fallah]

My query is when and why conventional PSV is required for subcritical flow behavior (i presume normally happens in system with high backpressure)

pravin,

Because contrary to ballanced bellows type, they have the capability of superimposed back pressure compensation by their spring setting adjustment.

Fallah

[latexman]

Pilot operated valves are usually supplied with filters to protect against foreign matter and are generally recommended for relatively clean gas, vapor, or liquid service There are applications where pilot operated valves are not recommended - dirty, corrosive, freezing, polymerization, coking, etc. Usually, rupture disks under pilot operated valves are not recommended. The pressure sensing line, even if remotely located, needs to be upstream of the rupture disk to sense the process pressure, and this defeats the purpose of the rupture disk.

In the cases where pilot operated valves cannot be used, the designer can manipulate set pressure of the PSV (and rupture disk, if needed) and maybe MAWP of the vessel (new design) so that constant and variable superimposed backpressure + built-up backpressure can be handled. In these cases, the 10% backpressure rule of thumb for conventional PSV can be set aside and a safe operating window that complies with Code may be possible. If this approach is taken, then K_b for the conventional PSV will be needed.

[fallah]

The "Alternate Sizing Procedure for Conventional and Pilot-Operated Valves in Subcritical Flow" (section 3.6.4 of API 520) as where the OP has raised his question is clearly applicable for both conventional and pilot operated valves.

[latexman]

fallah,

Get that chip off your shoulder, dude. I did not say the method was wrong for POSVs.

I addressed the OP's "when and why conventional PSV is required for subcritical flow behavior". There are situations where a POSV should not be used, even though it is the best theoretical choice in a clean application, because it would be a maintenance problem and most likely would not function when required in a dirty service. I hope the OP saw I was indicating that good engineering judgement is more than using the right theory and equation from a book. It is using the knowledge we gain in our experience to not repeat things that did not work out well in the past.

Edited by latexman, 28 June 2012 - 08:38 AM.

[fallah]

Latexman,

By what i mentioned in post No.6 i didn't mean you said: "the method is wrong for POSVs". I just did want to redirect the discussions on the track which OP did initiate regarding sections 3.6.3 and 3.6.4 of API 520. Otherwise, your statement in post No.5 regarding one of the features of pilot operated and conventional PSV applications other than included in such sections of API 520 is completely right.

Fallah

[pravin164]

Dear Fallah & Latexman,


As said by "Latexman" conventional PSV can also be used for systems having high backpressure (superimposed + Built-up) where POSVs have problem as mentoned above i.e. in dirty, corrosive services by changing the spring setting for superimposed backpressure and using Kb value.

As i understood from "API code" as well as from "Fallah" that PSV sizing for systems having high back pressure can be eneded up with the subcritical flow bahavior method. In this case if the system allows only to use conventional PSV then could we ignore the code's requirement of max built-up backpressure i.e. 10% of set pressure.

Pravin..

[latexman]

pravin164,

The definitions and nomenclature on back pressure, and my casual use of them in my above post, has led to a misunderstanding. My apologies! Let's straighten this out. We are focusing on built-up back pressure, which is only one component of the total back pressure. No, built-up back pressure for conventional PSVs in compressible service cannot exceed 10% of set pressure. Said another way, the discharge line(s) of a conventional PSV in compressible service must be sized such that frictional losses do not exceed 10% of the set pressure to pass its discharge to atmosphere.

[pravin164]

Dear Latexman and Fallah,

Further to this discussion going on i am littel bit confuse about the term built-up backpressure. As for as my understanding the built-up back pressure is back pressure at PSV outlet nozzle, generated while PSV is operating at full opening(i.e. frictional losses with rated flow of PSV).
Now can we analyse this term with following set of example data for more clearity on the term built-up back pressure...

Suppose

1. PSV set pressure Ps=2 kg/cm2 (g)
2. Superimposed back pressure (constant) = 0.3 kg/cm2 (g) may be LP header pressure
3. Releiving scenario is some process upset (10% accumulation)
4.Eventually due to physical properties of relieving fluid the PSV sizing method ended upto subcritical flow behavior and i am intended to use conventional PSV.

Now as discussed above ideally i should reset the PSV spring for constant superimposed back pressure and use Kb value for the same reason for cpacity correction and size the outlet line within the codes requirement of max 10% built-up backpressure i.e. Frictional losses.

Query: Here in this case although we are maintening the 10% built-up back presssure criterion but the actual PSV back pressure is 0.5 kg/cm2 (g) all the time.The same is more than allowable accumulation also. Is this a deviation from codes requirement.

Note: The codes requirment of max allowable backpresssure for conventional PSV is best fit for the cases wherein PSV discarge line routed to atmosphere (Built-up backpressure = total backpressure) and no superimposed back pressure.

[fallah]

pravin,

In your example total back pressure in the case of PSV relieving at full lift conditions is 0.5 kg/cm²g as you mentioned, but for a part of this value (0.3 kg/cm²g) as constant superimposed back pressure the compensation has already been done by spring setting adjustment. Hence, at the moment the PSV is starting to be opened at its set point (zero accumulation) there is 0.3kg/cm²g pressure at PSV downstream and it is adequate the build-up back pressure wouldn't exceeded to 10% of the PSV set pressure; means to be lower than 0.2 kg/cm²g.

Fallah

[latexman]

The "shop" would set the differential set pressure at 1.7 kg/cm2 (g). With constant superimposed back pressure of 0.3 kg/cm2 (g), the PSV set pressure (i.e. will open at) is 2 kg/cm2 (g). Built-up back pressure/set pressure x 100 = 0.2/2 x 100 = 10%.

[pravin164]

PSV outlet will face the total value of back pressure all the time when it is in relieving condition. Also total back pressure (Built-up + Superimposed) in absolute terms is used in all the applications of back pressure (Pb) for PSV sizing methodology.
Then why the API code requires back pressure limitation for built-up back pressure only.

Pravin...

[fallah]

pravin,

Because superimposed back pressure (maximum one for variable) wouldn't depend on psv tail pipe size and configuration and being fixed at early design stage of the relevant project then normally cannot be changed by any later modification, while build-up back pressure absolutely depends on the tail pipe size and route/isometric and on the other hand come into play when PSV is in operation where maximum allowable pressure in PSV upstream is equal to accumulation.

Fallah

[rahul22yadav]

Dear Mr. Pravin164,
As per your querry and its discussion i understood that you are trying to say that for high back pressure conventinal PSV's are used. But than my question is why the back pressure limitation for conventional PSV's is only 10%. I am not understanding this concept.

[fallah]

Hi,

Build-up back pressure limitation value for conventional PSV's is maximum relevant allowable overpressure that would be 10% of the set pressure at 10% allowable overpressure.

For constant superimposed back pressure the limitation would be the point that the PSV's spring load can still be reduced to compensate for the superimposed back pressure. Obviously, the PSV size would be increased for higher superimposed back pressure for the same relief capacity.

Fallah