Relief Valve Trim

My questions relate to multiple relief scenarios on relief valves. I have attached Table 1 and Table 2 from API 520, Pt II below. In sizing relief devices we all most always have more than one scenario. Let us assume one scenario is liquid and one scenario is vapor, both need to be evaluated for capacity.

 

1. Do I choose the trim according to which scenario will require the largest orifice or since multiple phases can go through the valve choose liquid? For the question, assume the vendor does not make dual certified trim.

 

2a. Case 1: Assuming we have an existing valve and the trim is vapor, according to Table 1, if the device is vapor certified and I need to evaluate the liquid capacity I should use non-certified equations – no issue, I believe I understand what to do. I would use the gas ASME Kd and ASME/API gas equations for gas sizing and the liquid ASME Kd and non-certified API liquid equation for liquid sizing.

 

2b. Case 2: Assuming we have an existing valve and the trim is liquid, according to Table 1, if the device is liquid certified and I need to evaluate the vapor capacity, API specifies to talk to the manufacturer for estimated capacity. Do I still use the gas ASME Kd and ASME/API gas equations for gas sizing and the liquid ASME Kd and ASME/API liquid sizing equation for liquid sizing?

 

 

2b. Case 2: Assuming we have an existing valve and the trim is liquid, according to Table 1, if the device is liquid certified and I need to evaluate the vapor capacity, API specifies to talk to the manufacturer for estimated capacity. Do I still use the gas ASME Kd and ASME/API gas equations for gas sizing and the liquid ASME Kd and ASME/API liquid sizing equation for liquid sizing?

 

Hi,

 

The matter is so clear...for liquid relief capacity estimation you can do as you mentioned but for vapor relief capacity estimation you should consult manufacturer...

Fallah,

 

Every time I talk to a manufacturer they are clueless about my question... namely Emerson is clueless. What do you do? What about Question 1?

Edited by ryn376, 08 May 2020 - 08:23 AM.

The relief sizing is based on the worst of:

• the worst case liquid scenario,
• the worst case vapor only scenario, and
• the worst case two-phase scenario.

It's that simple.

But if I have a two scenarios, one has and one liquid, and a liquid rated valve which kd do I use for sizing the liquid and which kd for gas? The liquid trim valve does not have the same kd as a gas trim valve.

Let me state in a different manner. I need to verify that a relief valve is adequately sized. The certified capacity is in gpm liquid with a certified liquid kd.

How do I estimate the gas flow rate (i.e. which kd value do I use)? The trim is not gas certified. The trim is not dual certified. The liquid trim is not used for gases, they switch the trim out for a different design for gases. Should I use the certified gas kd even though it doesn't have gas trim? If so, why is that acceptable?

Don't get confused by the facts.  Drill down to the worst case and size with that.  Use the certified gas kd for the gas case and the certified liquid kd for the liquid case.  The one which requires the largest area is the worst case.  Put the worst case info into the spec and use the worst case kd, be it a gas kd or a liquid kd.  The other case will fit within the specified relief.  No worries!

Don't get confused by the facts.  Drill down to the worst case and size with that.  Use the certified gas kd for the gas case and the certified liquid kd for the liquid case.  The one which requires the largest area is the worst case.  Put the worst case info into the spec and use the worst case kd, be it a gas kd or a liquid kd.  The other case will fit within the specified relief.  No worries!

 

I agree...!

 

but, I will go one step ahead and specify the governing case for Liquid, Gas, Two Phase rather than specifying the overall governing case.

Yes, kinda, I document inlet and outlet pressure drop for the worst case gas, worst case liquid, and worst case two phase in the relief record.  If I'm selecting the relief, the vendor only sees the worst worst case.  If I'm letting someone else select the hardware, I give them the whole record.