9 replies to this topic

[kinetic123]

Hi all,

 

I have few queries based on attached specs for relief valve for two phase flow [water + steam].

Please note that we have received these specs from supplier but confused as there are so many K's....

 

Queries

1. What are K, Kd [discharge coefficient?], Kw, Kb [back pressure coefficient?] and Kc and where should I use them?
2. It seems that K Gas is 90% of Kd Gas and K Liquid is 90% of Kd Liquid. Why so?
3. Why rated capacity is 90% of actual capacity?
4. For inlet and outlet pressure drop calculations, which flowrate do I need to consider i.e. is it rated or actual capacity? I believe, I need to consider rated capacity [9980.002kg/hr] ?
5. I am calculating maximum two phase flow capacity for the required relief valve using Leung’s method using formula G = C_D * h_fg / ( v_fg * SQRT ( C_f T_o)). What discharge coefficient, C_D [here it might be either Kd Gas or Kd Liquid?] should I use?

Thanks

Attached Files

•  Specs.docx   172.6KB   20 downloads

[Bobby Strain]

The vendor will have the answer to your query. As will API recommended practices.

 

Bobby

[breizh]

Kinetic 123,

 

This handbook should help you , if not consider to talk to the vendor.

 

Added a calculation note.

 

Hope this helps

 

Breizh

Edited by breizh, 29 April 2014 - 12:26 AM.

[aroon]

Queries

1. What are K, Kd [discharge coefficient?], Kw, Kb [back pressure coefficient?] and Kc and where should I use them?
2. It seems that K Gas is 90% of Kd Gas and K Liquid is 90% of Kd Liquid. Why so?
3. Why rated capacity is 90% of actual capacity?
4. For inlet and outlet pressure drop calculations, which flowrate do I need to consider i.e. is it rated or actual capacity? I believe, I need to consider rated capacity [9980.002kg/hr] ?
5. I am calculating maximum two phase flow capacity for the required relief valve using Leung’s method using formula G = C_D * h_fg / ( v_fg * SQRT ( C_f T_o)). What discharge coefficient, C_D [here it might be either Kd Gas or Kd Liquid?] should I use?

 

Before reading datasheet, you should read API-520 and API-521 carefully to understand the terminologies involved in relief system analysis and their significance. Here are the short answer to your question, however, I would suggest you to read API and very nice handbook uploaded by Breizh in his above post:

 

1. K, Kd are the discharge coefficients and those values are different for different phases (Gas/vapor, liquid and two phase). Preliminary values of Kd for sizing per API are 0.975 (for Gas/vapors), 0.62 (liquid non certified valves) and 0.65 (for liquid certified valves), 0.85 (for two phase mixture).

 

Kw and Kb are the back pressure coefficients, the significance of this factor is consider the effect of back pressure on the capacity relief device.

 

Kc is the combination capacity factor, which generally 1.0, however, if you have rupture disc at the inlet of relief valve it should be accounted based on resistance coefficient of the rupture disc (based on the type).

 

2. and 3. Manufacturer should be consulted for those.

 

4. General approach is to calculate the pressure drop on:

- Actual capacity for fluid with Gases/vapors/two phase at the inlet of relief device. It is actually due to pop-up action which tend valve to open fully irrespective of required capacity.

- Required capacity for fluid with liquid phase at the inlet of relief device. This is due to linear/proportional opening characteristic of with respect to required capacity.

 

5. Why Leung's method? Why you are not using universally accepted HEM method suggested by (DIERS).

 

 

Hope this helps you. 

[kinetic123]

Thanks Bobby, Breizh and Aroon,

 

@Aroon,

 

Thanks for your detailed explanation. Will refer to quoted standards in details.

Your last point regarding Leung’s & HEM method – I have few queries

 

Please let me know if my below understanding is correct.

 

 

1 I  have one release scenario in which heat is transferred from the reactor content [ say at 250degC] to the jacket full of cooling water at 10 degC [jacket outlet blocked]. It appears that it will give two phase release as jacket is full of water and sudden temperature rise.

 

2. For the release case stated in point#1 above, I am using PSG8 PartC which directs me to use Leung’s method. Leung’s method is used to find required minimum vent area [A in m2] using mass vent capacity [G in kg/ m2 sec]. Here, G is to be determined by ERM [simplified equilibrium rate model] method. With ERM method let’s say G is about 6000kg/m2 sec while by HEM model, the G is about 4000kg/m2 sec.

 

This means, HEM model is more conservative compared to ERM model?

 

3. Am I correct in saying that the REQUIRED two phase release rate for the release case stated in point#1 above is simply A x G? If so, I believe terminology for G used in PSG8 is wrong – Shouldn’t it be ‘Required two phase mass vent capacity’ rather than just mass vent capacity? OR am I missing anything here?

 

Thanks in advance.

Edited by kinetic123, 30 April 2014 - 10:14 AM.

[mrbabu]

I have not direct experience with PSG 8 but Leung's method is discussed in API

(see Omega, simplified HEM methodology), the Equilibrium Rate Model (ERM) is

similar to HEM but with N parameter set to 1.0 and that means that in most cases

it is less conservative,

for detailed discussion see Emergency Relief System Design Using DIERS Technology of similar documents.

 

About your point 3 there are examples of two phase flow solved with Leung/Omega in API,

you can refer to those examples in case of doubts,

usually one calculates the area required for the specified flow, then

select the most suitable model from catalogs and repeat calc's,

several manufacturers provide specific software which can include

two phase flow, with these you can verify your results,

as alternative download a copy of Prode or similar software which

includes HEM & HNE models.

Edited by mrbabu, 30 April 2014 - 12:54 PM.

[aroon]

Hi,

 

I guess explanation by "mrbabu" is sufficient to clarify your above queries.

 

However, I have one basic question before moving forward to select sizing method to calculate required area. Since the case is off thermal expansion of cooling water due to heat liberated from reactor.

 

"What is the relief temperature at the inlet of relief device?" Just I would like to check the condition of the fluid (water) at relieving pressure i.e. sub-cooled liquid, flashing/saturated liquid, or vapor at the inlet. This has a major impact on selection of sizing method and "Kd".

 

Be careful while selecting "Kd" for "sub-cooled liquid or flashing/saturated liquid". Blindly use of two phase API Kd (0.85) is sometime less conservative.

[kinetic123]

Aroon,

 

Thanks very much for your reply.

I have considered relieving temperature of 168.3deg C which equates to steam pressure of 7.6bara [also can be seen on the attached specs sheet in the original post]. Not sure, how’s its going to affect Kd. Please comment.

 

As per, PSG8, the Kd for a relief valve is to be considered same as that of gas/vapour i.e.  a frictionless nozzle for two phase flow – please see below statement taken from PSG8.

 

“A safety valve can usually be treated as a frictionless nozzle, when determining the two-phase flow capacity. The same discharge coefficient as is recommended for gas flow should be used”

 

As you stated, Kd for two phase flow is 0.85 as per API 520 Part I Appendix D. This means API 520 again gives more conservative value NOT less conservative? I am sure, I haven’t missed anything here.

 

However, me myself not convinced how is it possible to treat relief valve as frictionless nozzle for two phase flow while it is the worse than liquid and of course gas release!

 

Thanks in advance again.

[mrbabu]

the Leung Omega model doesn't include the contribute of friction,
it is a simplified model based on a linear function for fluid density (see the original paper),
with rigorous thermodynamic models (I have Prode)  you can, for example, include a loss coefficient to account for both the entrance loss and pipe friction loss but these values are not easy to evaluate.
For steam/water there are many data sets in literature (I have that compiled by Sozzi and Sutherland) which you may use as reference.

Kd is the discharge coefficient, API gives the values for preliminary sizing, the effective value should be from tests.
Finally, consider that for some conditions HNE or NHNE (which take in account the rate of heat and mass transfer between vapor and liquid) may give better results than HEM / Omega.

[kinetic123]

Thanks mrbabu...