10 replies to this topic

[aju_1807]

Hi All,

I have one small query which many times has been raised by my collegues many times. I hope somebody can throw some more light on this issue.

Well, we have a slug catcher where the sizing case for the relief valve is Blocked outlet case (4P6-3 nos) and one of the non-governing case for the slug catcher is fire for which required orifice area is very less (coming roughly G type). However, as the selected orifice is of P type, the rated flow rate for fire case (corresponding to P type) is coming very huge (due to higher difference between reqd orifice area and selected orifice area). Now, it has been questioned many times in our group that whether relief valve will able to pass rated flow rate (in fire case) or not?

I know the ideal solution for this would be to use multiple valves with staggered set pressure and

[fallah]

Hi All,

I have one small query which many times has been raised by my collegues many times. I hope somebody can throw some more light on this issue.

Well, we have a slug catcher where the sizing case for the relief valve is Blocked outlet case (4P6-3 nos) and one of the non-governing case for the slug catcher is fire for which required orifice area is very less (coming roughly G type). However, as the selected orifice is of P type, the rated flow rate for fire case (corresponding to P type) is coming very huge (due to higher difference between reqd orifice area and selected orifice area). Now, it has been questioned many times in our group that whether relief valve will able to pass rated flow rate (in fire case) or not?

I know the ideal solution for this would be to use multiple valves with staggered set pressure and

As long as relief load of fire case (or each non governing case) would be more than 25% of relief load of governing case,there is no problem such as chattering and duration of relief would be very short (with respect to governing case relief).Otherwise chattering will be occured.

Edited by fallah, 24 August 2009 - 05:47 AM.

[ashetty]

Hello Aju,

I do not understand your query.As you Presented it...

Governing case = Blocked outlet, Orifice size 4.P.6 (Selected orifice area ~ 6.4 sq inch)
Non governing case= Fire case, Orifice size..lets say: 1.5.G.3 (Selected orifice area ~ 0.5 sq inch)

Now your required orifice area is calculated based on your required relief load.
Now for the sake of example let us assume for fire case your required relief load is 2100 kg/hr for which your required orifice area is 0.32 sq inch.
This required size is larger than the orifice area for designation 1.5.F.2 which is 0.31 sq inch, so you go to the next size which is 1.5.G.3 (0.5 sq inch)
Rated flow for required flow of 2100 kg/hr would be (2100*0.5)/0.32, roughly 3300 kg/hr

Now for orifice size P the required flow would be ten times that of orifice designation Q....

Anyway for the rated flow depends mainly on the orifice size.

Please send data and i can explain better

Regards,

[ashetty]

Hi Aju,

just another query.

Also what is the reason for calculating rated flow for fire case corresponding to P type? P type requirement is for blocked outlet relief load.

Regards

[fallah]

Hi Aju,

just another query.

Also what is the reason for calculating rated flow for fire case corresponding to P type? P type requirement is for blocked outlet relief load.

Regards

P type is selected type for governing case,as per post originator's statement.
Thus it is obvious orifice area of P type is applied for calculating relief load of all other nongoverning cases.

Edited by fallah, 25 August 2009 - 01:27 AM.

[ashetty]

Hi Aju,

just another query.

Also what is the reason for calculating rated flow for fire case corresponding to P type? P type requirement is for blocked outlet relief load.

Regards

P type is selected type for governing case,as per post originator's statement.
Thus it is obvious orifice area of P type is applied for calculating relief load of all other nongoverning cases.

Hello Fallah,

Why should we use selected orifice type of governing case to calculate relief load of non-governing case?

Am i really missing something here?

Regards,

A.Shetty

[fallah]

Hello Fallah,

Why should we use selected orifice type of governing case to calculate relief load of non-governing case?

Am i really missing something here?

Regards,

A.Shetty

In engineering stage non-governing cases result in lower orifice areas,but finally largest orifice area would be selected.

After installing the selected PSV (as per governing case e.g. blocked outlet) all non-governing cases (e.g. fire case) faced with larger orifice area lead to having higher (governing) relief load (but with shorter time duration) with respect to what were considered in engineering stage.

[aju_1807]

Thanks for your reply.

In my case as fallah mentioned, we have fire case a non-governing case and the rated flow rate through P type PSV (which is already selected) will be huge due to sinificant difference in required orifice area for fire case and selected orifice area (i.e. of P type). In this case we can the rated flow rate for fire case will be momentarily but is it required to size the tail pipe for the rated flow rate?

We have proposed following two options:

1. Size the first relief valve for fire case (i.e of lower orifice) and size other PSV's with staggered set pressure for BO case.
2. Use of modulating releif valves which will only allow required flow rate to pass through relief valevs (which is really costly option).

One of the clause in API-521, table-12 says that some of the spring loaded conventional pressure relief valves have modulating capabilities. So it will allow required rate to pass through in the event of fire. we have raised the query to RV vendor and awaiting his reply on this issue.

Have anybody has used such kind of releif valves in your any of teh job?

My basic concern is that i have to increase the line size for fire case rated flow rate (which will be momentarily) so is it required to size for rated flow rate?

Appreciate your prompt response.

[fallah]

In this case we can the rated flow rate for fire case will be momentarily but is it required to size the tail pipe for the rated flow rate?

As long as you use one conventional PSV for all scenarios,you should consider rated flowrate of worst case for sizing the tail pipe.

[S.AHMAD]

Thanks for your reply.

In my case as fallah mentioned, we have fire case a non-governing case and the rated flow rate through P type PSV (which is already selected) will be huge due to sinificant difference in required orifice area for fire case and selected orifice area (i.e. of P type). In this case we can the rated flow rate for fire case will be momentarily but is it required to size the tail pipe for the rated flow rate?

We have proposed following two options:

1. Size the first relief valve for fire case (i.e of lower orifice) and size other PSV's with staggered set pressure for BO case.
2. Use of modulating releif valves which will only allow required flow rate to pass through relief valevs (which is really costly option).

One of the clause in API-521, table-12 says that some of the spring loaded conventional pressure relief valves have modulating capabilities. So it will allow required rate to pass through in the event of fire. we have raised the query to RV vendor and awaiting his reply on this issue.

Have anybody has used such kind of releif valves in your any of teh job?

My basic concern is that i have to increase the line size for fire case rated flow rate (which will be momentarily) so is it required to size for rated flow rate?

Appreciate your prompt response.

Dear Aju

I think you are in the right direction for considering the two options. However,I do not understand why you need to increase the line size for the fire case? If I understand the arguments correctly, the fire case requires smaller size relief valve (G) that means the flowrate due to fire is much smaller then that of blocked discharge (size P). Is my understanding correct? If it is correct then this means the line size is more than sufficient for fire case. I suggest you adopt option 1 since it is the cheaper option. Set pressure can be staggered as per API RP. A word of caution, use the right accumulated pressure for sizing the fire case to ensure that it opens and remains at a lower pressure and does not accumulated to the set pressure of the bigger RV. Otherwise the bigger RV will chatter.... if my understanding is not correct then my comments is no longer applicable.

Best regards

S.Ahmad

[aju_1807]

We have raised query to RV vendor to provide the modulating capability for the conventional spring loaded relief valves (as some of the spring loaded relief valve also have modulating capability and in such cases its not required to sizxe tail pipe for rated flow rate). We are awaiting venodr response but at the same time we have provided relavant details of fire case relief load to Asset management group in our organization to check the acoutic induced vibration in the line (as Mach no is reaching 1 for fire case).

If the acoustic study favours the lower line size with sound power level less than 180 dba(thresold value) then we have to size the outlet line based on governing case of RV only.

I am still awaiting reply from both parties. I will surely update you once I find appropriate solution on this issue. As per API521 and also as per standard philosphy tail pipe should be sized for rated flow rate so there is no argument that whether we should size the outlet based on rated flow rate or not.

Thanks everybody for your valuable comments and feedback.

Regards,

Ajay