Gas vessels: Operating Pressure used: 4

[jamesbanda]

The purpose of this question is to understand partices used for operating pressure Management of change and relief load evaluations.

For a gas filled vessel the relief rate / ultimate pressure rise depends on the start pressure (e.g.) how much gas is present at the start. So. From a relief sizing perspective, if you have a higher start pressure the relief valve will be larger.

So, what is the convention that users are deploying for the start pressure.
a) Max, normal operating pressrue (high alarm)
b) Software based trip
c) Hardwired based trip.

Clearly, if the trip point is used the relief valve is larger but more management of change is considers prior to changing this than an operating point. If an operating point is selected one could unknowingly change the operating pressure without knowing and find the relief valve is undersized.

 

[MortenA]

I cant just see why the PSV should be larger if the initial pressure (before fire) is higher or lower?

The relief load will depend on heat input and gas heat capacity. The heat capacity shall be calculated at relief conditions - so initial conditions wont matter and the heat input is constant. As far as i can see a relief valve will be the same. There may be a lag time before the valve pops - but thats not important here.

 

[jamesbanda]

If you work through the formulas you find it does impact the sizing.

• Step 1: Calculation of maximum Pressure rise from gas expansion. (is a relief valve required for gas expansion)
Pmax = Pop *(Tw/Top)

• Step 2: Determine the area required to pressure relief in a fire

o Calculate vessel wetted area

o Calculate Relief temperature in a fire (This does up the higher the start pressure)
The temperature of the relief is based on P1/T1 = P2/T2

o Calculate constants F’ and C

o Calculate Area required for relief valve.

• Step 3: Calculate relief valve

 

[zdas04]

I just ran through a scenario that a vessel is half full of water at 40 psig and a second where it was at 400 psig (both with a 600 psig MAWP). I got essentially the same required relieving rate for both scenarios--if there was still liquid in the vessel at MAWP then it was at the same temperature in both scenarios, and if the liquid was all boiled away then I never get to the PSV setting in either case because the metal failed before PV=nRT raised pressure to the MAWP).

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[don1980]

James - I assume you're referencing the API 521 equation for fire exposure to a vapor filled vessel. That equation is based on a false premise that a PSV can protect such vessels from fire exposure. Unlike all the other equations in that document, this equation isn't underpinned by objective logic. It's arbitrary, just as the results generated from this equation are arbitrary. Thus, there's no value in considering the effects of one starting pressure over another, assuming this equation is sensitive to that (I didn't investigate).

Reactive relief relief sizing can be affected by the initial pressure in the vessel, but for non-reactive scenarios the PSV size is independent of that value. PSVs are sized based on the relief requirement at the relieving pressure, regardless of the path it took to get there.

 

[jamesbanda]

Don,

Thanks for the email.

Ok, so it is arbitary, for most EPC firms they do use this equation to generate a relief rate so it is used fairly widely i believe (and have seen).

So if you have a gas filled surge drum what basis do you assume for the relief valve, and how do you prove it is effectively sized?

 

[zdas04]

You size it based on a "credible scenario" analysis. You size all pressure safety devices based on a credible scenario analysis. That analysis is totally outside the PSV calculations and is specific to the actual use of the vessel. As an operator I should go through the categories of credible scenarios in API 521, select the ones that are possible in my system, determine if the possible scenarios are credible, and for the ones that are determine the flow rate that the system can provide at relieving conditions. The largest flow rate (or combination of flow rates in rare cases) of a credible scenario should be used in pressure safety device sizing. You don't get to the API 521 calculations until the very last step.

I'm not sure what you mean by "prove it is effectively sized". You start with a few thousand assumptions, roll them into a flow rate, and size your valves based on that flow rate. Every one of the assumptions is subject to be wrong, and those errors can be material. The only "proof" in this process is documentation of your assumptions so that they can be assessed in subsequent reviews.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[don1980]

James – People naturally assume that methodologies published in API 521 are technically sound and have consensus endorsement from technical experts. I think that that confidence is justifiable for everything in this standard with one exception – the topic we’re discussing. This section in API 521 (5.15.2.2.2) is highly misleading in that the reader gets the impression that a PSV can safely protect a vessel from fire exposure when that vessel is filled with a vapor or supercritical fluid. That’s simply not true. Regardless of the PSV size, the vessel wall temperature continues to rise unabated. Unlike vessels containing a boiling liquid, there’s no “heat in from fire = heat out through PSV” equilibrium established at the relieving pressure. Although imperfect, that’s the theoretical mechanism by which PSVs protect vessels (those containing a liquid) from fire exposure. That doesn’t occur, and can’t occur, when there’s no boiling liquid in the vessel. The heat capacity of a gas/vapor is trivial. Consequently, during a fire there’s insufficient heat transfer between the vessel wall and the vessel contents (heat that’s released through the PSV). The result is a rapid increase in the vessel wall temp, and without the necessary cooling (Hvap) the vessel quickly exceeds its yield stress. Even if the internal pressure increases enough to pop the PSV (in many cases the yield stress is reached before the PSV can even open) that PSV will quickly reclose without having released any significant amount of heat. That’s true regardless of the PSV size. Even a large PSV won’t release enough heat to have a noticeable effect on the vessel wall’s rate of temperature rise.

The API 521 vapor expansion equation has been in that document for many years, and since it has been widely used over those years, there’s probable significant inertia for keeping it. But, notice that 5.15.2.2.2 has a number of statements which hint at the problem discussed above, and notice the subtle way the text says you “can” use the equation, without indicating that this methodology is recommended or that it provides safe protection.

Personally, I think it’s indefensible for API 521 to discuss this tangentially rather than directly. Meanwhile, many companies recognize the futility of using a PSV for protecting vapor-filled vessels from fire. A PSV is needed for other credible scenarios or for code compliance if there are no other scenarios, but there’s no technical justification (or code mandate) for sizing such PSVs for fire exposure. What the engineer should do, in my opinion, is openly point out that the fire exposure is a risk which can’t be mitigated by a PSV, and that the owner should perform a risk assessment to determine if other protective measures are needed. Those include auto depressurization, fire resistant insulation, and NFPA15 water-spray. In some cases the risk may not justify any of those, while in other cases the risk may justify using one or some combination of those protective measures.

 

[jamesbanda]

Thanks Don,

This is very helpful. I have come accross many vessels that fire is the only case.

So, have you seen vessels in the fire zone (gas filled) that dont have a relief valve and this is allowed under codes ?

 

[don1980]

This is a common situation that I run into often. You still have to install a PSV for code compliance (or apply UG140). What I’m saying is that it’s pointless to size that PSV for fire. Size the PSV for another credible scenario – the one that requires the largest orifice.

If you find that there are no other credible scenarios (that’s pretty common too) then the PSV size is arbitrary – you can simply choose a size. All pressure vessel codes intentionally leave the sizing decision to the user.

Engineers will often make up a scenario, just to have something to calculate, but that’s also pointless and misleading. State in your documentation that there are no credible scenarios, thus no sizing calculations - there’s nothing on which to base the sizing. Choose a PSV size. I generally use a 1D2 unless it’s an exceptionally small or exceptionally large vessel. If it’s real small, then I use a 3/4x1. If it’s very big, then I might choose a valve slightly larger than a 1D2. Next, proceed with checking the inlet and outlet piping based on the rated capacity of that valve. Note that you don’t need a scenario in order to know the valve’s rated capacity.

 

[BenThayer]

my concern for gas only vessels protected by a PSV or for liquid vessels that boil off the liquid and become dry is that if the fire goes long enough, eventually the vessel fails and bursts.

if that is considered to be "credible" then you might be better off with a PSD. "might" is a key word because i would not consider that for highly hazardous chemicals such as chlorine, etc.