6 replies to this topic

[narain82]

Hi,

 

We say that a modulating pilot PSV opens enough only to relieve the required flow rate. Let's say a modulating pilot PSV is installed on a vessel overhead and caters for fire case as well as non fire case.

 

The required relieving flowrate for the fire case is calculated considering 121% accumulation. The required relieving flowrate for the non fire case is calculated considering 110% accumulation.

 

The question is whether the modulating PSV will open such that it relieves the required flowrate during non fire case and will maintain 110% accumulation. And will the same modulating PSV during fire case will open to relieve only the required flow rate and maintain 121% accumulation.

 

Thanks in advance for all your thoughts.

[fallah]

narain82,

 

It depends on the govern case...

 

If a non fire case is govern case, full lift would be occurred at 110% of the set pressure and other cases including fire case are to be done in partial lift with lower overpressure than 10%... 

 

If fire case is govern case, full lift would be occurred at 121% of the set pressure and other cases are to be dine in partial lift with 10% overpressure or lower...

[narain82]

Fallah,

 

Thanks for your reply. But I could not interpret your points to visualize the working of a modulating pilot PSV which is catering to multiple scenarios. I still have the following queries:

 

1. In any case when iam preparing a PSV datasheet do I need to specify the required flowrates calculated using 110% for non fire cases and 121% for fire case ?

 

2. The flow through the PSV will never be more than the required flow rates (during respective scenarios i.e fire or non fire) as specified in the datasheet?

 

3. In your reply you have used the term "Full lift". In this case "Full lift" means the required orifice area ? or the actual available PSV orifice area?

 

4. An example with some figures will help me in understanding the concept which you are trying to explain.

 

Thanks.

[narain82]

Fallah,

 

Carrying forward your explanation I am trying to write down my understanding. Please correct me wherever necessary.

 

Non fire case 1: Required relief rate = 9000 kg/h (for 110% accumulation), calculated orifice area is 4.71cm2 (for 110% accumulation) , Installed orifice area is 5.06 cm2

Non fire case 2: Required relief rate = 4000 kg/h (for 110% accumulation), calculated orifice area is 2.09 cm2 (for 110% accumulation) , installed orifice area is 5.06 cm2

Fire case: Required relief rate = 5788 kg/h (for 121% accumulation), calculated orifice area is 3.55 cm2 (for 121% accumulation), installed orifice area is 5.06 cm2

 

The PSV has an actual area of 5.06 cm2. However the modulating PSV will be designed such that at 110% accumulation, the valve lift will be 4.71 cm2 (based on the non fire case 1 which is the governing)

 

When the PSV pops up due to the non fire case 1 the accumulation will be 110% with an open area of 4.71 cm2. The relieving rate will be 9000 kg/h.

When the PSV pops up due to the non fire case 2 the accumulation will be less than 110%. The open area will be more than 2.09 cm2. The relieving rate will be 4000 kg/h.

When the PSV pops up due to fire case the accumulation will be less than 121%. The open area will be more than 3.55 cm2. The relieving rate will be 5788 kg/h.

 

Similarly, if the fire case turns out to be governing then the accumulation during fire case pop up will be 121%. However, during other non fire cases the accumulation will be less than 110%. The actual orifice open area for the non fire cases will be more than the calculated orifice area.

 

Thanks all. Would appreciate all your view point on all my posts above.

[Bobby Strain]

Don't concern yourself with such trivial details. Instead of asking a bunch of questions, you should learn how a modulating pilot works. I am certain your supplier will have literature on the subject. And, when you purchase a relief valve you need to include enough information to size the valve and to select materials for the various parts. It's not too difficult.

 

Bobby

[fallah]

narain82,

 

The modulating POSV will relieve its rated capacity (not governing required relief load) at full lift (rated orifice area) and 10% overpressure (governing fire case 21%)...

 

For the govern case and the cases other than governing case, the relevant required relief load will be relieved at partial lift (required orifice area) and 10% overpressure or lower...

[breizh]

http://www.wermac.or...ure_relief.html

 

Hi ,

May be good to read ?

 

Breizh