15 replies to this topic

[omnibus]

I need to calculate pressure drop at the downstream pipes of Safety/ Relief valves for light hydrocarbons ( ethane, propane, ethylene etc). The down stream pipes of various PSVs are about 2 to 10 meters in length connected to a common flare header.
The question is

Will it be OK to consider each PSV D/S line independent of others and calculate pressure drop for single phase (gas)?

Will it be OK to consider the condition steady state for the calculations?


The above is required to judge the back pressure due to the downstream lines of the various safety/relief valves.

Thanks in advance.

[latexman]

You need to consider credible scenarios. Some will involve one PSV only, and one might involve all the PSVs. If it's credible, you gotta consider it. Are all the vessels, these PSVs are on, in the same "fire area"? Are they in the same dike for containment? Or, are they in the same area sloped for drainage to a remote area? Or, are they in separate fire areas. How long do you expect the fire scenario to last? It makes a big difference. Do you expect the fire to last long enough for PSVs to "pop"? If so, and 2 phase flow is credible, you have to consider it. If your fire protection measures are superb and a group/team of informed experts agree the PSVs should not "pop" (at least that's how we do it) in the worst case, credible scenario, you may be justified in sizing the PSV(s) with vapor only flowing, thus meeting Code.

[pleckner]

1. The tail pipe of each individual PSV should be sized using the stamped capacity of that PSV. It doesn't really matter what the controlling scenario is.

2. However for common headers and manifolds, sizing is generally based on the worst-case cumulative required capacities of all devices that can reasonably be expected to discharge simultaneously. In this case, you do use the flow from the controlling scenario for each PSV. As Latexman pointed out, fire case is typical where you may expect multiple reliefs going at the same time. ASME "suggests" that the common header/manifold have a cross sectional area at least equal to the total of all the individual tail pipes associated with the simultaneous discharges.

Whether you can use just gas or you need to use two-phase is a matter of whether you determine if you have two-phase relief or not. We cannot say yes or no.

Steady state is the usual way to proceed with the caviet of flashing liquid or condensing of vapor as you travel the flow path and pressure drops. Again, only you can determine if your system unfortunately requires this.

[omnibus]

1. The tail pipe of each individual PSV should be sized using the stamped capacity of that PSV. It doesn't really matter what the controlling scenario is.

2. However for common headers and manifolds, sizing is generally based on the worst-case cumulative required capacities of all devices that can reasonably be expected to discharge simultaneously. In this case, you do use the flow from the controlling scenario for each PSV. As Latexman pointed out, fire case is typical where you may expect multiple reliefs going at the same time. ASME "suggests" that the common header/manifold have a cross sectional area at least equal to the total of all the individual tail pipes associated with the simultaneous discharges.

Whether you can use just gas or you need to use two-phase is a matter of whether you determine if you have two-phase relief or not. We cannot say yes or no.

Steady state is the usual way to proceed with the caviet of flashing liquid or condensing of vapor as you travel the flow path and pressure drops. Again, only you can determine if your system unfortunately requires this.

Thanks for the replies. I think I need to be more specific.

There are 50 PSV's fitted on various vessels/ lines, tailpipes of which are connected to flare header. It has been proposed to instal isolation valves ( gate) D/S to PSVs so that a particular PSV can be isolated for maintenance/ replacement.

My task is to calculate the pressure drop caused by the Valve and extended length and additional elbows warranted by installation of the isolation valves. I need to be reasonably accurate as the excess backpressure caused by this pressure drop needs to remain under the limits prescribed for the various PSVs.

The first trial calculations will be for the existing line size and resizing will be required only if the pressure drop is more.

Will it be OK to use Darcy's formula assuming incompressibility in such cases? Has anybody done a similar calc?

Another thing is --|Can I use the gas properties ( density and viscosity) at given Release Pressures of various PSVs considering that the length of the piping is relatively small in all the cases?

Moreover, will it be ok to use simulation software like Flownet (incompressible gas flow scenario), for this purpose?

Thanks in advance for assistance.

[Wendy]

I have done some PSV back pressure calculations. What I normally use is Darcy's formula but do it stepwise, establish equivalent pipe lengths between points in the flare system based on the specific piping layout.

Start at the flare pit where its at atmospheric pressure or at the flare skid edge providing the skid back pressure is known. Use the design flow to work towards the individual relief valves. Take into account the loss of pressure produced by fittings, such as elbows and reducers. Then calculate the inlet pressure for each section of the flare line by adding the calculated pressure drop for that section to the known outlet pressure (the inlet pressure of a calculated section was used as the outlet pressure for the new section). Working towards the PSV by repeating the above.

Mach number need to be checked also.

Hope the above helps

[pleckner]

Thanks for the replies. I think I need to be more specific.

There are 50 PSV's fitted on various vessels/ lines, tailpipes of which are connected to flare header. It has been proposed to instal isolation valves ( gate) D/S to PSVs so that a particular PSV can be isolated for maintenance/ replacement.

My task is to calculate the pressure drop caused by the Valve and extended length and additional elbows warranted by installation of the isolation valves. I need to be reasonably accurate as the excess backpressure caused by this pressure drop needs to remain under the limits prescribed for the various PSVs.

The first trial calculations will be for the existing line size and resizing will be required only if the pressure drop is more.

Will it be OK to use Darcy's formula assuming incompressibility in such cases? Has anybody done a similar calc?

Well yes, I've done this similar calculation many times.

What I said before still holds true for your situation. The only difference is that you are doing a rating rather than a design. As I said before, for each PSV tail pipe, you must use the PSV rated capacity for your flow. For the header, you must still know all possible simultaneous releases and you may use the actual relieving capacity for each valve instead of the stamped capacity. You must still check the entire system and not just concentrate on the individual tail pipes as the header adds to the system backpressure. Again, nothing has changed from what I and Latexman said before.

Using Darcy, I'm not a big fan as the Isothermal gas flow equation is just as easy to use and will check for choked flow as part of the calculation. Darcy can only be used if the pressure drop is not greater than 10% of the upstream pressure. You can still use Darcy up to a pressure drop of up to 40% of the upstream pressure but you would then need to use the average vapor density. Using a gas flow equation eliminates this problem and you won't have to break the line(s) into segments. And if you do use Darcy, you still need to check for choked flow. Saying this, if the tail pipes are very short and sized properly, you will probably be within the 10% "rule", won't have to worry about choked flow and you should be able to get away with Darcy. However I would still use the gas equations because then there is no guessing.

You should indeed be using the properties at the outlet of the PSV. If you use the gas flow equations (Isothermal or Adiabatic), that's all you'll need.

Any commercial software package dealing with fluid flow will work. But why use the incompressible equations? If the software is any good, it should have the proper equations for compressible flow as well.

So in summary and repeating, nothing has changed from what I and Latexman said before.

[omnibus]

I have done some PSV back pressure calculations. What I normally use is Darcy's formula but do it stepwise, establish equivalent pipe lengths between points in the flare system based on the specific piping layout.

Start at the flare pit where its at atmospheric pressure or at the flare skid edge providing the skid back pressure is known. Use the design flow to work towards the individual relief valves. Take into account the loss of pressure produced by fittings, such as elbows and reducers. Then calculate the inlet pressure for each section of the flare line by adding the calculated pressure drop for that section to the known outlet pressure (the inlet pressure of a calculated section was used as the outlet pressure for the new section). Working towards the PSV by repeating the above.

Mach number need to be checked also.

Hope the above helps

I think that is enough information for me. Thanks a lot, Latexman,Pleckner, Wendi!
Pleckner, I plan to crosscheck the results by Flownet or HYSYS th spredsheet calcs. Have you used HYSYSor ASPEN PLUS for this sort of calcs? I plan to use hysys for single pipe but the problem is that I have to specify either pressure or temperature at the downstream of the pipe, either of which are not known. Do you have any sugessions? Thanks , as always are in advance.

[pleckner]

No, I haven't used either for these type of calculation. I have my own spreadsheet but I've also started to move into SuperChems (Relief Systems design software by Iomosaic) and the programs given on the CD that comes with my favorite resource, "Guidelines for Pressure Relief and Effluent Handling Systems", Chemical Center for Process Safety of the AIChE, 1998.

One thing I've done in the past is take the fluid at relief and do an isenthalpic expansion calculation using the downstream (final) pressure. This is not necessary the best but it has usually been good enough.

However recently I've moved toward a more accurate method, and much more time consuming. I would do a series of isentropic flashes to calculate my PSV. That's the more time consuming part. Once I have the choked mass flux at the PSV throat I start with the properties of this fluid and will do the isenthalpic expansion calculation. This way I start with the more correct upstream fluid properties, i.e. enthalpy. But again, if you use SuperChems, it does this for you.

[omnibus]

No, I haven't used either for these type of calculation. I have my own spreadsheet but I've also started to move into SuperChems (Relief Systems design software by Iomosaic) and the programs given on the CD that comes with my favorite resource, "Guidelines for Pressure Relief and Effluent Handling Systems", Chemical Center for Process Safety of the AIChE, 1998.

One thing I've done in the past is take the fluid at relief and do an isenthalpic expansion calculation using the downstream (final) pressure. This is not necessary the best but it has usually been good enough.

However recently I've moved toward a more accurate method, and much more time consuming. I would do a series of isentropic flashes to calculate my PSV. That's the more time consuming part. Once I have the choked mass flux at the PSV throat I start with the properties of this fluid and will do the isenthalpic expansion calculation. This way I start with the more correct upstream fluid properties, i.e. enthalpy. But again, if you use SuperChems, it does this for you.

Hi All,

I am finally in aposition to start. The only remaining doubt is that in most of the datasheets I received from the client , 'Required Capacity' is given. In very few cases actual or rated Capacity is also given. API 521 part ii , page-9, 5.2 states that the tail pipe sizing should be done on the basis of Rated Capacity. then it also mentions that for a modulating pilot operated pressure relief valve the discharge pipe sizing can be done on the basis of Required Relief Capacity of the protected system.

Now we know that the actual capacity is expected to be slightly more than the required capacity and the tail pipe sizing should be according to the maximum flow it is expected to receive.

So the question is; Should I insist on my demand for Actual Rated Capacity figures for the various PSVs or is there a way to know it even if it is not given in the datasheet. If I am not wrong it is expected to be stamped by the vendor.

May be some fo the questions are too 'stupid' but this is my first task of this type and I am depending on you druids for guidance.

Thanks as always, are in advance.

Now

[pleckner]

Rated capacity is often (but of course not always) a lot more than required capacity. And yes, you need to insist on having the rated capacity of the PSV. If you know the model of valve that will be purchased you can get the rated capacity from the vendor's catalogue and/or website. If you don't know the model yet then have the client give it to you.

The rated capacity will be stamped on the PSV tag but in terms of the test medium, i.e. air or steam. You will then need to convert this to your specific gas equivalent. ASME gives this conversion. Or, you can use vendor software to do the job. Farris has one as does Anderson Greenwood, etc.

[omnibus]

Hi Pleckner,

MAYDAY again. The client says it is OK with him if the calculations are done based on Required flow as given in the older data sheets. The data for actual relief capacity is not available. Will it make much difference if I base the calculations on the Required Relief Capacity as calculated.

Another option is that the actual orifice areas are available. Can you tell a reliable method of flow calculation through PSV orifice?I haven't done it before so please be a little elaborate if possible. Will be highly obliged.

[pleckner]

omnibus:

The criteria for line sizing as I outlined hasn't changed and neither will my statement about it. Whether there is a significant difference in using required relieving capacity (for the individual tail pipes) or the stamped capacity really depends on the ratio of actual orifice size to required orifice size. The closer to "one" it is the less significant the difference will be.

Now you totally confuse me when you say

Another option is that the actual orifice areas are available.

If you have the actual orifice then you must know what the actual PSV is (make and model) so you will know the stamped capacity of the valve as well. What information are we (or you) missing here?

To obtain required orifices, I strongly suggest you get a hold of API RP 520 ASAP!! AND, if you haven't done these type of calculations before, or are just a novice at it then I also strongly advise you have a more experienced engineer look over your shoulder and don't take this upon yourself. This is no place where you can afford to get it wrong.

[omnibus]

Hi Pleckner,

I have got API 520/521 available from which I have the allowable overpressures, backpressures etc for various types of PSVs ( there are all three types , conventional. pilot operated and balanced bellows , in my case)
You are right. I haven't done such calculations before. It is a matter of somebody showing the way once but where I am working is a dog-eat-dog place and I do not expect any help from anybody except selfless netgurus like you guys. It is matter of leading by hand once--.only once.

What I am expected to do is :
It is not the sizing but it is to conform the back pressure due to flow in the tail pipe up to end point (flare tip)
I have to calculate pressure drops in the tailpipes of various PSVs upto flare for each PSV. There are some combination PSVs for which I will take total capacity and in some cases most likely simultanious discharge will be taken into consideration.

What I have are the datasheets of the PSVs and I am still collecting the isometrics of various lines up to flare. As per the client the information for most of the PSVs actual capacity is not available. Though orifice size and model information is available in the datasheets, I was unable to get the actual flow from most datasheets.The PSVs are from, Anderson Greenwood, FUKUI and Crosby. Now in each datasheet calculated area and actual selcted orifice area are given. Can I take the actual flow to required flow ratio as the ratio of actual area and calculated area? If yes it solves the problem of actual flow.

What will be the way to calculate pressure drop after I have the full pipe/fittings/elevation data? I think I am able to see the way but can you please lead by hand just once. Will be highly obliged. And I was wrong in stating in some previous mail that the DP is needed only for a short portion of the piping which is rerouted. Now I realise that I have to do it from PSV discharge to flare tip in each case.

[pleckner]

I'm obviously having trouble communicating to you and for this I apologize. You state:

Though orifice size and model information is available in the datasheets, I was unable to get the actual flow from most datasheets.The PSVs are from, Anderson Greenwood, FUKUI and Crosby. Now in each datasheet calculated area and actual selcted orifice area are given.

The actual flows won't necessarily be shown on the datasheets but you can get them from the vendor literature. If you know the model number, just go to the vendor's website, look up the PSV and you will find the actual flow of that PSV in-terms of air (most likely) or steam if they are steam relief. You would then need to convert this into your gas/vapor equivalent. ASME provides the simple equation in Section VIII, Div 1 Appendix 11.

11-1
The capacity of a safety or relief valve in terms of a gas or vapor other than the medium for which the valve was officially rated shall be determined by application of the following formulas:

For air:

(1) Wa = CKAP sqrt(M/T)

C = 356

M = 28.97

T = 520 R

For any gas/vapor:

(2) W = CKAP sqrt(M/T)

Wa = rated capacity, converted to lb / hr of air at 60°F, inlet temperature
W = flow of any gas or vapor, lb / hr
C = constant for gas or vapor which is function of the ratio of specific heats, k p cp / cv (see Fig. 11-1)
K - coefficient of discharge [see UG-131(d) and (e)]
A = actual discharge area of the safety valve, sq in.
P = (set pressure × 1.10) plus atmospheric pressure, psia
M =molecular weight
T = absolute temperature at inlet (°F + 460)

You know Wa from the vendor literature. Find KAP (P is the test pressure; usually but not always the same as the relieving pressure).

Substitute KA back into the equation using your gas/vapor M, T, P and C to get the equivalent W.

Or you can call your vendor rep and he/she can get you the information.

And yes, you can ratio the actual area to the calculated area to get the "actual" flow IF the calculated area was based on the actual coefficient of discharge and the "actual" area you have on the datasheet is the ASME area.

To get the pressure drops, refer back to the previous posts where you were given the technique to use. Note that this is nothing more than a standard gas/vapor fluid flow problem that I would expect any practicing chemical engineer to be able to perform. You can reference your text book on fluid flow, Perry's Chemical Engineering Handbook or better yet, Crane TP410.

Saying all this, if you have any specific questions on performing the pressure drop calculation, don't hesitate to post them here.

[Sean C...]

It was indicated that the piping hydraulics should be calculated at the rated capacity for the Relief Valve. Please consider that this is a derated value (Kd) and the valve will in fact flow on average 10% more flow. On smaller valves and depending on the manufacturer, a valve could flow 30% more flow that what is indicated.

[pleckner]

The value being referred to by @Sean C is commonly called the "best estimate flow rate". This flow should be used to design the downstream processing units, it is not required for the piping design per standards. Some people will use the best estimate flow for the piping as well and this is just a more conservative design, not necessarily a better design. It is always best to check with your company standards to see how the powers-to-be want the system sized.