4 replies to this topic

[pika_doh9]

Question 1: I am sizing the PSV for a Air Receiver Vessel

 

API521 Section 5.15.2.2.2 says the effective discharge area of the valve is A = Fx A"/(P)^0.5

 

where A is the discharge area os valve, A" is the surface area of vessel, P is the upstream relieving pressure and F is the environmental factor

 

However in API 520 part 1, section 3.6.2.1.1 sizing for critical flow, it has another equation for effective discharge area.

 

The equations from API521 gives me a bigger orifice size to API520 - why is this and which one am I meant to use?

 

 

Question 2: I am doing a PSV sizing for a MEG storage tank

 

I have chosen the case thermal expansion due to fire...if the PSV is opened, will it be MEG liquid flow or will it be the Nitrogen blanket that comes out through the valve or a mixture of both / 2 phase flow release case

 

 

 

 

 

 

[fallah]

Hi,

 

About question 1: Please submit the sizing detail

About question 2: In fire case on a storage tank, normally an emergency vent is being considered to handle the relief load. Please clarify and submit further information

[Lowflo]

pika -

 

Q1: The API 521 equation you cited is one for fire exposure to a vapor filled vessel. That equation has been in APi521 for many years, but there's no technical justification for it. Personally, I think it should be removed because it's misleads, and it's bad advice to users of that document. It misleads readers into thinking that a PSV can protect such vessels from fire exposure. The truth is that a PSV can't do that when there's no liquid in the vessel to absorb the fire heat, and that liquid will boil at the relieving pressure. A vapor filled vessel is going to fail when exposed to fire, regardless of the size of the PSV. Size the PSV for a scenario other than fire. If you don't have any other credible scenarios for this vessel, then just stick a small PSV on it for code compliance. No sizing calculations are necessary if you don't have a sizing basis (credible scenario).

 

Q2: Assuming this is a low pressure tank and not a pressure vessel, follow API 2000 (or EN 14015 if the tank is in Europe) for tank vent sizing. MEG is a Class IIIB liquid, which means it's not normally considered a source of fire. Determine whether there are flammable liquids in the area before proceeding with fire sizing. If there are no flammable liquid in the area, then follow the procedures for sizing a breather vent rather than a fire-sized ERV. If there are flammable liquids then size an ERV in addition to the breather vent. The referenced standards have clear instructions on how to do that.

[Bobby Strain]

If there is nothing flammable nearby, there is no need to protect the air receiver against overpressure due to a fire.

 

Bobby

[Olidin]

Question 1:

 

Well, API kinda of explain it in API 521 5.3 last paragraph:

 

In such cases, the assumption of an ideal gas can be too conservative, and Equation (8) (see 5.15.2.2.2) oversizes the pressure-relief valve. This equation should be used only when physical properties for the fluid are not available.

 

If you have all the fluid properties (sounds like you do to know it's critical flow), then use API 520 Part 1. 

 

However, I think that equations in API 521 are meant for calculating required relief rate where API 520 was meant for calculating capacity. So even though the equation you cited in API 521 give an erroneous orifice size, it does give you a required relief rate later on with that orifice (eq 12). Again, it's an estimate and could be wrong. 

 

Also, I have had air receivers with an external fire scenario before since air can expand before vessel is rupture. (for some gases, this is unlikely) so I can see how you consider external fire here. However, if you didn't know that, check out API 521 5.15.1.2 "Effect of fire on the unwetted surface of a vessel"

 

Question 2:

 

Like falah said, not enough info here. Since he answered the low pressure version, I'll take the high pressure version. Though here are the questions

 

1. Why would there be a thermal expansion for this storage tank (vessel)? What is the storing temperature. 

2. Note, an external fire would be the controlling scenario here (I think). So spending time calculating hydraulic expansion is really a waste of time. I would suspect that thermal expansion rate would be much lower than vapor generation rate due to fire. 

Edited by Olidin, 11 October 2013 - 03:53 PM.