21 replies to this topic

[kv.balu]

Hi all

I am designing a Safety valve relief outlet piping handling saturated steam at 270 Deg.c. Set pressure of the valve is 50.0 kg/sqcm and the size of the safety valve is 4 x P x 6. Required Relieving capacity of the valve is 91 tph. The valve has been already selected and to prevent a bellows safety valve, the consultant had specified a built-up back pressure of 5 kg/sqcm(g), which is 10% of set pressure. The problem now is about the size of the relief piping. As the valve is open to atmosphere, it would be built-up back pressure on the valve when it is relieving. I am trying to calculate whether the backpressure is less than 10% of set pressure.

 

Request members on how the back pressure could be calculated considering a 6"piping (with two 90 deg elbows) of about 3 metres.

 

Is the back pressure on the relief valve, the same as the Static pressure at the outlet of the safety valve when the valve is relieving ? or is it the difference between the static pressure at outlet of the safety valve and the static pressure at outlet of the tail pipe? Or is it the stagnation pressures due to flow that one should consider.

 

Kindly post your views

 

regards

 

[fallah]

kv.balu,

 

The build up back pressure is the static gauge pressure at the PSV outlet when is relieving, and with mentioned configuration you described appears the build up back pressure cannot reach to 5 kg/sqcm(g), but it should be confirmed by calculation of pressure drop in the PSV discharge line along with the choked flow evaluation through the PSV and subsequent tail pipe...

Edited by fallah, 26 August 2014 - 11:57 AM.

[kv.balu]

fallah,

 

My calculation works out about 10.2 kg/sqcm(g) static pressure at outlet of Safety valve.  Should I not be considering the pressure drop (about 4.3 kg/sqcm) in the tail pipe as the Backpressure on the safety valve.  Kindly clear the doubt.

 

regrds

[alexlucas333]

Is it static or stagnation pressure at PRV outlet flange?  This has been something thats always kind of confused me. Is it not just the calculated DP from friction losses?

[latexman]

alexlucas333,

 

That works most of the time, but that simple approach cannot be used when the flow can choke in the pipe size of the PSV's outlet flange.

[Bobby Strain]

You need to confirm that 10% is correct. If the valve is protecting a steam generator, the allowable accumulation is less than 10%. So the backpressure, too, will be less than 10%. And your calculation method appears correct. Simply calculate the pressure at the outlet flange of the relief valve.

 

Bobby

[kv.balu]

Bobby, 

Thanx on your post.

 

The overpressure of 10% specified is as per the data sheet of the consultant.  Values are as below:

 

Size of tail pipe - 6"

Length - 6m including 2 90 deg 6" LR elbows

 

Flow - 91 tph of steam

set pressure - 50 kg/cm^2

 

Safety Valve outlet

Static pressure - 9.2 kg/cm^2

Stagnation pressure - 11.68 kg/cm^2

 

Tail pipe end

Static pressure - 4.87 kg/cm^2

Stagnation pressure- 9.473 kg/cm^2

 

What constitutes back pressure on the safety valve?

 

regrds

[fallah]

kv.balu,

 

Can you upload the calculation you performed to obtain the static pressure at the PSV outlet during relief? It's needed to evaluate the result you submitted in post no#3...

[latexman]

This is a known dilemma in the relief design world.

 

Reference: Guidelines For Pressure Relief and Effluent Handling Systems, Center For Chemical Process Safety of the American Institute Of Chemical Engineers, 1998, pages 38-39.  “On certain occasions, the designer can encounter a dilemma when sizing a tail pipe to meet the built-up back pressure limitation for conventional pressure relief valves. Calculations for a tail pipe of the same size as the outlet flange can say that the built-up back pressure exceeds the allowable limit even if the length is reduced to zero.”

 

For gas flow, this problem usually arises only for valves with small ratios of outlet-to- nozzle area and rather high set points, like the 4P6 in this case.  I do not know of any test data nor evidence in  industrial experience for PSVs with no tail pipe to show that a real operational problem is indicated.  For flashing two-phase flow, the problem can arise at higher area ratios and lower set pressures.  Test data on saturated water obtained in the DIERS program showed that a serious loss of capacity and stability can occur as the ratio of outlet to nozzle area is reduced below that of a standard 2J3 valve (Sallet and Somer 1985). The set pressure for these tests was about 90 psig.

 

Using kvbalu’s case as an example, an Engineer sizing a 4P6 PSV with no tailpipe simply sees no problem because there is no need to calculate backpressure.  However, an Engineer sizing a 4P6 PSV with tailpipe sees the problem because they must calculate backpressure.

 

The problem does not arise unless the calculated flow from the valve nozzle is predicted to be high enough to attain choked-flow conditions in the pipe size of the PSV’s outlet flange. But, choking is not the problem. The problem arises only if the outlet flange, choked-flow pressure exceeds the allowable back pressure limit for the particular valve, such as this case.

 

The problem can appear to go away if a larger tail pipe size is specified without proper modeling of the flow in the pipe expansion. The dilemma remains nevertheless - if the back pressure is too high even with zero length of outlet size pipe, the back pressure will not be reduced by addition of pipe of larger size, right?

 

The dilemma then is whether or not to change the PSV to obtain calculated results in accord with the non-mandatory Code requirement (select a valve or valves with higher outlet flow area for the given nozzle flow area).  And, in this particular case, I don’t think a P nozzle PSV with a higher outlet flow area exists.  It is quite a dilemma.

 

So, what to do?  Definitely, work with the vendor’s “expert” or technical support.  They have seen this issue before.  Can you eliminate the tail pipe?  Can you reduce the set pressure to the point where the backpressure equals the accumulation?  (I calculated about 12% backpressure).  Can you use two smaller PSVs?  Can you modify/change to bellows PSV?  Can you use a Pilot PSV?

 

Kvbalu2129, you have a very interesting problem here.  Please come back with more questions and let us know how it is resolved.

[latexman]

kv.balu,

 

Looks like there are some discrepancies between your posts #1, #3, and #7.

 

Is the tailpipe 3m or 6m long?

 

Is the tailpipe end static pressure 4.87 kg/m2 (#7) or 5.9 kg.cm2 (10.2-4.3 in #3)?

 

These discrepancies must be rectified before the big issues can be dealt with.  Maybe a meeting with the consultant would be beneficial.

[kv.balu]

The Piping is about 3 Metres long.  Thanx. am attaching the output.

Attached Files

•  CFF.pdf   38.64KB   176 downloads

[kv.balu]

Bobby, 

 

referring to your post at #6, i am attaching a safety valve vendor's calculation for static pressure calculation at outlet.  Does this value have any significance when outlet vent piping is fixed.

 

                    Calculation of Outlet Static Pressure at Safety Valve                             Set Pressure, Pset     50 kg/sqcmg 49.35 barg     Overpressure, Pover   3% 1.5 kg/sqcmg 1.48 bar     Backpressure, Pback   10% 5 kg/sqcmg 4.93 bar     Inlet Loss, Pinlet     0 kg/sqcmg 0.00 bar     Atmospheric Pressure, Patm   1.0332 kg/sqcma 1.013 bara     Safety Valve outlet diameter   154.05 mm 15.41 cm     Area at outlet, Ao         186.39 sqcm     Orifice Area         41.16 sqcm     Relieving Pressure, P1=Pset+Pover-Ploss+Patm   51.84 bara                       Kd Discharge coefficient   0.98       Kn Napier correction factor   1.00       Ksh Superheat correction factor   1.00       Ksc Superheat correction factor   1.00       Kc Rupture disk CCF     1.00       Inlet stagnation enthalpy, ho       2792.90 kJ/kg                       Outlet static pressure, Po =(0.01812*A*P_1*kd*kn*ksh*ksc*kc*((h_0-1914.3)^0.5/A_o))-Patm   Outlet static pressure, Po       4.98 barg                    

[kv.balu]

if the above was unreadable, pls see attachment

Attached Files

•  SafetyValve_Outlet Static Pressure calculation.pdf   41.28KB   200 downloads

[latexman]

It looks like someone specified exactly 10% OP, and the vendor is regurgitating the numbers back.  It is valuable if the tailpipe installation has backpressure less than or equal to 10% OP, which in your case may not be true.  In any case, a tailpipe backpressure calculation must be done to see if it is equal to or less that 10% OP.  This attachment is not a tailpipe backpressure calculation.

[kv.balu]

Latexman, 

 

Calculation for the static pressure at outlet of safety valve (4.98 barg) is provided by a vendor in their PSV sizing sheet and would definitely not reflect the backpressure due to the tailpipe. The vendor did not definitely have any idea about the tail pipe configuration. I posted that calculation just to know its significance.

 

The earlier file (cff.pdf) contains the output report of the pressure profile along the tail pipe.

 

Request comments on both the posted files from the online communite

 

regards

[kv.balu]

Members,

 

Would an Orifice (with orifice dia as the safety valve P orifice) and an angle globe valve Plus an expander (4 x 6) mimic a 4 x P x 6 Safety valve?

 

regards

[kv.balu]

fallah,

 

Static pressure at outlet of safety valve is in the range of 9.0 - 9.5kg/sqcm while at the tail pipe exit is around 4.5-4.8kg/sqcm in either of the cases.  

 

Isnt backpressure just the pressure differential (pressure drop) between the ends?

 

I also have been trying out on eng-soft SVVent software. The output warns me 12 kg/sqcm backpressure, which is the stagnation pressure value at outlet flange of safety valve

 

Amazing similarities !! 

[Marc-Andre Leblanc]

Hello,

 

First of all , I will assume the PSV application you have is for a pressure vessel under ASME sec. VIII div 1. if its under a different code then your allowable backpressure will not be 10% (10% overpressure case) and the following calculations will be meaninglesss because allowable backpressure for Pressure vessel under ASME Sec. I (power boiler is not the same as under ASME sec. VIII.

 

 

Second, work in gauge pressure, the 10% allowance for backpressure is based on a gauge pressure fraction not an absolute pressure fraction, this is extremely important as your set pressure decreases. ( See exemple C.2.3.2.1 in API 520)

 

I will work with PSIG as a unit for pressure for the following calculaiton, its also easier to work with these units since both relevant standard use this unit 

 

Set pressure : 711 psig (will disreguard fractions as they are within the manufacturer limit for precision on set pressure)

 

Orifice P

 

fluid : Saturated Steam

 

Pressure at max relief conditions (10% overpressure) : 782.1 psig

 

Temperature at max relief conditions  (10% overpressure) : 517.9°F

 

Equivalent lenght of Pipe (considering 6" SCH 40) : 46.2 ft

 

Inside diameter of Pipe : 6.065 in

 

Rated Flow for P orifice : 291 700 lb/hr ( Back pressure calculation are normally performed at rated flow)

 

Resulting Back-pressure : 134,6 psig (based on calculation/ simulation)

 

Backpressure % = 134.6 psig / 711 psig = 18,9 % 

 

Resulting backpressure in 6" pipe is higher than the maximum allowed as per API 520 Standard.

 

Lesson : its not because the facing of your PSV is a certain diamether that your Piping need to be same size, 95% of the time you need at least 1 diameter higher if you want to use a conventionnal PSV and respect the backpressure allowance

 

WIth 8" pipe , equivalent lengh around : 57.9 ft

 

Resulting back-pressure = 68 psig (based on calculation/ simulation)

 

Backpressure % = 68 / 711 = 9.5%

 

So according to the calculation I made with the tool at my disposal a 8" pipe would be required to respect the 10% allowance

 

If you prefer not to change the piping, the option could also be to change the PSV to a balanced-bellow design, further calculation are required if that is the option selected

 

 

(Edit: Regarding the backpressure value used by the manufacturer

 

PSV manufacturer will not calculate your actual backpressure but they normally require that information when they do their design. This is because they need to verify if the flow is ctritical or non-critical trought the orifice. It is more conservative in this case to select the maximum back-pressure, this can result in a non-critical flow) 

 

Edit 2: Added Emphasis on confirmation needed that pressure vessel fall under ASME sec. VIII div 1 and not ASME sec. I 

 

Regards

 

Marc-Andre

Edited by Marc-Andre Leblanc, 27 August 2014 - 02:30 PM.

[Bobby Strain]

I viewed the last calculation. And, I suggested earlier that you confirm the allowable backpressure. And you did not do so. The calculation shows overpressure to be 3%, which suggests that this valve is protecting a steam generator. So, you must limit the backpressure to 3%, not 10%. You should engage a more qualified consultant. You need always to start at the beginning, and assure that your premise is correct.

 

Bobby

[fallah]

kv.balu,

 

In your case in which the flow will be choked going through the PSV and then goes to the atmosphere through a short tail pipe, it's better the PSV back pressure as well as the pressure drop in the discharge tail pipe to be calculated by Fanno lines calculations...

[latexman]

kv.balu,

 

I think Bobby Strain is onto another issue with this PSV.  What is the applicable Code and Division?  ASME?  Div. I or Div. VIII or other?

 

I agree with Fallah on fanno lines.  That is about as rigorous as you can get.

Edited by latexman, 27 August 2014 - 02:40 PM.

[kv.balu]

Fallah/Latexman,

 

The output report is considering Adiabatic flow with Friction (Fanno lines, isnt??)

 

Bobby,

 

Overpressure specified IS 3% (ASME Sec I).  I am putting up your observation on limiting backpressure also to 3% to the consultant.  The steam generator is operational and PSV was taken up as a check case due to constant leakage through the seats ( PSV vendor had deducted 5 kg/sqcm backpressure on the setpressure to 45 kg/sqcm, made a temperature correction and specified the CDSP as 45.45kg/sqcm).  The steam generator was operating at 43-45 kg/sqcm.!!!