11 replies to this topic

[ezralh]

 Hi All,

 

I have multiple PSV in 6.2 barg steam line. The PSV arrangement as in the attachment. The high alarm is at 7.24 barg. My questions are:

 

a) Why the PSVs are arrange as such?

Can I switch the PSVs? I put the 7.58 barg PSV before 8.27 barg PSV?

Attached Files

•  MulPRV.pdf   559.64KB   128 downloads

[fallah]

ezralh,

 

What is the value of design pressure of the steam line? Are they installed under ASME Sec. I?

[Pilesar]

The total required relief area for this service is evidently very large and is satisfied by two 6Q8 relief valves. The order or arrangement is likely of no consequence if the inlet piping is similar. Multiple pressure relief valves allow higher overpressure (16%) than a single relief valve. Only one valve is required to be set at or below MAWP with the other valve set up to 5% over the MAWP. The reason you have two PSVs may be documented in the sizing calculations. Possible reasons are total cost, available flanges, reduction in chattering, etc.

[ahmadikh]

 ezralh,

 

As Fallah said, please provide the design pressure. As far as I remember, PSVs under ASME I are not allowed to have any valves on their upstream, even LO/CSO.

Regarding your first question, I think these valves are located on the MP steam header after the let down valve (right? correct me if I am wrong) and hence there are two valves to share the loads and there is no difference which one to put first.

 

Best,

Milad

[ezralh]

ezralh,

 

What is the value of design pressure of the steam line? Are they installed under ASME Sec. I?

 

Fallah,

 

6.2 barg. I guess so.

[fallah]

 

ezralh,

 

What is the value of design pressure of the steam line? Are they installed under ASME Sec. I?

 

Fallah,

 

6.2 barg. I guess so.

 

 

ezralh,

 

I think 6.2 barg is operating pressure of the steam line and the design pressure of this line might be a value between 7.58-8.27 barg (set pressure of PSVs). Please clarify this point and specify if PSVs are governed by ASME Sec I...
 

[Lowflo]

ezralh - It's perfectly OK to switch the physical location of these two PSVs.  The current placement was obviously and arbitrary decision - there's no technical  reason for the current placement.

[mohammad reza]

Would you please let me know your technical points about below issue?

 

PSV load in different scenarios:

 

1) Cooling water failure: 600 kg/ hr

2) Fire case: 17000 kg/h

3) Reflux Failure: 23000 kg/h

4)abnormal heat : 3000 kg/h

 

in order to avoid chattering, i have to consider Multiple PSV or pilot operated -modulated type. in case of select multiple PSV i want to know:

 

1) Multiple PSV's Load 

2) PSV's set pressure

3) Spare PSV load and set pressure

4) accumulation pressure of each PSV

 

Thanks in Advance.

[fallah]

Would you please let me know your technical points about below issue?

 

PSV load in different scenarios:

 

1) Cooling water failure: 600 kg/ hr

2) Fire case: 17000 kg/h

3) Reflux Failure: 23000 kg/h

4)abnormal heat : 3000 kg/h

 

in order to avoid chattering, i have to consider Multiple PSV or pilot operated -modulated type. in case of select multiple PSV i want to know:

 

1) Multiple PSV's Load 

2) PSV's set pressure

3) Spare PSV load and set pressure

4) accumulation pressure of each PSV

 

Thanks in Advance.

 

mohammad reza,

 

1) Multiple PSV's load: The load of governing case among four mentioned credible scenarios

 

2) PSV's set pressures: Set the first (normally smallest one) at %100 of MAWP of the relevant vessel or lower and set the last (normally largest one) PSV at %105 of the set pressure of the first valve and set the other valves inbetween

 

3) Spare PSV load and set pressure: Same as the first PSV

 

4) Accumulation pressure of each PSV: For the first PSV the accunulation would be maximum %110 of the vessel MAWP and for the last valve would be maximum %116 of the vessel MAWP and for other valves inbetween
 

Edited by fallah, 05 July 2016 - 03:14 PM.

[mohammad reza]

 

Would you please let me know your technical points about below issue?

 

PSV load in different scenarios:

 

1) Cooling water failure: 600 kg/ hr

2) Fire case: 17000 kg/h

3) Reflux Failure: 23000 kg/h

4)abnormal heat : 3000 kg/h

 

in order to avoid chattering, i have to consider Multiple PSV or pilot operated -modulated type. in case of select multiple PSV i want to know:

 

1) Multiple PSV's Load 

2) PSV's set pressure

3) Spare PSV load and set pressure

4) accumulation pressure of each PSV

 

Thanks in Advance.

 

mohammad reza,

 

1) Multiple PSV's load: The load of governing case among four mentioned credible scenarios

 

2) PSV's set pressures: Set the first (normally smallest one) at %100 of MAWP of the relevant vessel or lower and set the last (normally largest one) PSV at %105 of the set pressure of the first valve and set the other valves inbetween

 

3) Spare PSV load and set pressure: Same as the first PSV

 

4) Accumulation pressure of each PSV: For the first PSV the accunulation would be maximum %110 of the vessel MAWP and for the last valve would be maximum %116 of the vessel MAWP and for other valves inbetween
 

 

Dear Mr. Fallah

 

1) The first PSV load will be 600 kg/h and second one is 22400 kg/h(23000 -600). your point.

2) I think it is not correct to Spare PSV load and set pressure should be same as the first PSV. Because for example in case of  abnormal heat scenario both PSV's shall be re-installed and should be calibrated in plant work shop, thus now we have a Spare PSV with capacity equal smallest scenario (600 kg/h) which could not protect equipment in case of for example fire or reflux failure at that time.

3) For multiple PSV 116% should be considered as accumulated pressure (As per API). My question about specify of accumulation for Spare PSV.  

 

Thanks 

Mohammad reza

[fallah]

 

2) I think it is not correct to Spare PSV load and set pressure should be same as the first PSV. Because for example in case of  abnormal heat scenario both PSV's shall be re-installed and should be calibrated in plant work shop, thus now we have a Spare PSV with capacity equal smallest scenario (600 kg/h) which could not protect equipment in case of for example fire or reflux failure at that time.

 

 

mohammad reza,

 

Actually if you have to consider one spare for multiple PSV's you have to consider same rated capacity for such multiple PSV's as well. Hence, the spare PSV should be set at the pressure same as the first operating PSV because if it being set at higher value, relieving may be started at a value higher than MAWP and this may not be acceptable in code limitations standpoints...
 

[mohammad reza]

Dear Mr. Fallah

 

I check standard and some company manual , but i did not find any rule for this issue. it seems when there are wide range load for PSV (same as my example) it is not mandatory to considering multiple PSV in order to avoid chattering phenomena. i think one PSV is enough with one spare.