Clarification of example given in API-520 2

[mucour]

Dear All,

I am a bit confused with the solution of the example given in API-520, PArt-1-Sizing and Selection (Sizing, Slecetion and Installation of Pressure -Relieving Devices in refineries.

In Example 4.3.3.2, page 31 (6th Edition, March 1993),

The problem I am having is that a total back pressure of 62.5psig was calculated and the set pressure of the relief valve is 75 psig. Based on my understanding a conventional relief valve cannot be used for an application if the back pressure is more than 10% set pressure.

Could somebody clarify to me why a convetional valve was selected in this example?

 

[grt12]

I suggest you get a more up to date copy of API 520 - the latest is January 2000.

 

[mucour]

Sorry, I do not have the latest version.

But the question still stands.

The example is the first example under the section (4.3.3) "Sizing for Subcritical flow: Gas or Vapour other than steam"

.


Thanks

 

[fdomin]

I don´t have an older version, but you should check values of superimposed and built-up backpressure.

For conventional PSV´s if superimposed backpressure is constant (i.e atmospheric) it will be considered for calibration of spring pressure, so it will not affect relief rate.
In the other hand, built up backpressure is added to the spring pressure and tends to reclose the valve, reducing relief flow. For that reason conventional valves are not recommended when built-up backpressure (not total backpressure) exceeds 10% of set pressure and bellows balanced PSV are prefered.
Hope this helps.

 

[TD2K]

The example that Mucour mentions appears to be similar/identical to example 3.6.3.2 in the Jan 2000 API 520 version.

3.6.3.2 in Jan 2000 has the required relieving flow as 53,500 lb/hr of mixed C4s and C5s (average MW is 65). Set pressure on PSV is 75 psig with 55 psig constant back pressure. Cold differential set pressure is therefore 20 psig on the valve.

The maximum allowable built-up back pressure is mentioned as 7.5 psi (10% of 75 psig set pressure) which gives a total built-up (constant plus allowable) of 62.5 psig, same as mucour mentions. There's another check that is done for the pressure drop ratio across the PSV which concludes that the sub-sonic capacity equations need to be used to size the valve.

As Mucour said, this isn't an application that I would have thought of 'conventional relief valve' (don't know if I would have gone with a balanced bellows also, the correction factor would be pretty low). A pilot valve would be preferred IMO.

I don't know the question to your answer Mucour, I'll have to do some digging but posted this in case someone else has it.

 

[TD2K]

Mucour, your confusion is due to the differences meant by different back pressure terms and their effects on relief valves in API 520. Look at the section titled "effects of backpressure on pressure relief valve operation and flow capacity" for the discussion on this. Built-up, superimposed back pressures and total backpressure all have very different meanings.

The 10% limit of set pressure for conventional relief valves applies to the built-up portion of the back-pressure, not the sum of built-up back pressure and superimposed back pressure. In the case you refer you, the example uses 7.5 psi as the maximum allowable built-up back pressure for a conventional valve (basically, they assumed that you would check and verify the actual is no more than this because they say you should check and verify the back pressures). With the 55 psig superimposed back pressure by the system, the total backpressure on the valve when relieving will be 62.5 psig. You can then use manufacturer's charts or the correction table(s) in API 520 In this case, the capacity correction factor would be about 0.86 using the sub sonic correction factor.

 

[mucour]

fdomin could have been a little bit clearer if he had considered the scenario when the superimposed backed pressure is, for example, 55 psig as in this example.

From API-520, it defines:
Sumperimposed back pressure as the static pressure that exits at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system coming from other sources and may be constant or variable.

Built-up back pressure is the increase in pressure in the discharge header that develops as a result of flow after the pressure relief opens.

These definitions are very clear.

From my interpretation, the built-up back pressure is a pressure resulting from the time when the relief device pops open and the flow create an impediment to the smooth relief of the vessel in the form of creating a back-pressure.

Whereas the superimposed back pressure is a function of the outlet of the relief valve. That is either open to atmosphere or the relief outlet is routed through a closed relief manifold such that there could be constant/variable back pressure in the manifold system resulting from other relief valves during simultaneous reliefs or resulting from back pressure from a vent scrubber that is also used for other purposes like continuous flare gas routed through the vent scrubber.

Example 4.5.2 is a case where the superimposed back pressure is variable 0 - 50 psig. The relief valve selected for this case is balanced bellow type relief valve. This makes sense to me.

I have read "effect of back pressure on pressure relief valve operation and flow capacity" suggested by TD2K. The question still stands.

Could there be other reasons why example 4.3.3.2 proposes conventional relief valve?

Thanks for the inputs.

 

[CHD01]

MUCOUR:

Regarding the API520 example 4.3.3.2 - this is my "take" on it:

Technically and in the real world I would agree with you that a balanced bellows valve is preferrable. If for no other reason than "how can they be sure 55 psig is REALLY CONSTANT!?! I would rather design for 0 to 55 psig variable back pressure.

That's point 1, now for point 2.

Again, technically a conventional valve is acceptable since (as you said) the 55 psig is constant, and 7.5 psig has been assumed to be the maximum variable without calculation (not a good practice except for preliminary sizing).
This is verified as follows: the total of built up back pressure (7.5 psi)plus the RANGE of constant back pressure (55-55 = 0) is 7.5 + 0 = 7.5 psi; and 7.5/75 is equivalent to 10% back pressure TOTAL.

This is OK since we have protected ourselves by setting the back-pressure derating factor to 0.88 (I used chart 30)in calculating the required orifice of 5.60 in2 (I actually get 5.63 in2 because my EXCEL sheet calculated C= 325 versus API's C = 326).

Point 3:

Again, in the real world, once I saw my design resulting in a back-pressure derating factor of < 1.0 ; that would normally trigger me to go back and select a balanced bellows valve.

Point 4:

If this was an existing valve, I would do it the way API has with the de-rating factor of 0.88.

Other Comments:

One thing I noticed that confuses me was that API in the Figure 30 footnotes implies (says?) this &quot;correction factor should be used only in the sizing of conventional (non-balanced) pressure relief valves that have their spring setting adjusted to compensate for the superimposed back pressure&quot;. I wonder if I really believe that, I believe in fact that the chart is used for all conditions of back pressure so long as its a conventional valve in vapor and gas service.

The rules of thumb I have used for back-pressure factors before I know what manufacturer and model I have is as follows (I think I'm consistant with my previous notes):

For Liquids

kw = 1.0 if < 16% backpressure or for non-bellows type
valves

For Vapors. Gases:

kb = 1.0 if < 30% backpressure (TOTAL) for Bal Bellows or if
< 60% backpressure (CONSTANT, i.e.
SUPERIMPOSED) for a
Conventional Valve

Finally, my usual rule of thumb is to use a Balanced Bellows valve anytime TOTAL back-presssure is > 10%. So, here again, I would never do what API520 has done EXCEPT where the valve is existing. THEN, its a matter of engineering judgement regarding best practices and cost of re-work IMHO.

The more you learn, the less you are certain of.

 

[mucour]

TD2K & CHD01,

I tried to read the chart, figure 30-constant back pressure sizing factor, K(b), for conventional safety relief valves, but what I read was not the same as what API-520 indicated.

Example 4.3.4.2 calculated the percent of absolute back pressure as 0.794 = 79.4%

Then it indicated that the, Back pressure correction factor, K(b), is 0.88 (from Figure 30).

But when I read the chart I got 0.79.

Could I be wrong, or I do not know how to read the chart well?

Please explain.

Thanks for the inputs.

 

[CHD01]

79.4% backpressure is correct if constant plus variable is included - which is what the API example does!

Using API520, FIg 30 @ k = 1.09 (curve not plotted) and extrapolating above the given curves - kb = 0.84 seems closer to what I would use than 0.88 for our example. I'm not sure how you get 0.79 unless you used the curve for k = 1.5. I found myself mis-reading the y-axis at first.

Looks like the dotted lines in Fig 30 is for the example included with Fig 30; not for our example.

I tried checking some of the vendors equivalent charts and got the following kb's:

FARRIS, kb = 0.85
CROSBY, kb = 0.854 (a table instead of a curve)
CONSOLIDATED, kb = 0.84

Now for some confusing factors:

CONSOLIDATED says to use the curve only for conventional valves with constant backpressure (doesn't discuss variable or built-up backpressure)

FARRIS = CONSOLIDATED (only constant backpressure considered for conventional valves)

CROSBY doesn't discuss it in the detail others do.

API says constant only also, but in their and our example they INCLUDE the max variable dp of 7.5 psi???

My company policy is to use max backpressure = constant plus built-up when determining kb for a conventional valve in vapor/gas service where constant super-imposed backpressure exists (this is what API did in the example).

I'm going to stick with my company policy which is equivalent to what API actually did on the example we discussed.

Now, are we all confused?

The more you learn, the less you are certain of.

 

[TD2K]

I have a headache ;-)

 

[mucour]

CHD01,

I have cross checked the K(b) value using CROSBY table (not chart) and it is okay.

But the chart in the API-520 (figure 30) is confusing.

Even if you follow the dotted line in figure 30 to the y-axis using their example of percentage absolute back pressure of 76% (see the example on the bottom of the chart), you will not get the K(b) API-520 stated as 0.89 from the curve of figure 30.

When I followed the dotted line, the K(b) is 0.78

This is also giving me headache. Sorry TD2K.

Any ideas CHD01.

Thanks

 

[CHD01]

I'm using API520-Part 1, 5th Edition, July 1990 and the dotted lines for Figure 30 on page 35 goes from 76 on the x axis to the curve identified as k = 1.3; from there the dotted line goes to the vertical y axis and that value is kb = 0.89. I don't know how you get 0.79, unless you have a different edition with a different plotted curve. The example for Fig 30 agrees with the plotted data in the edition I'm using. The more you learn, the less you are certain of.

 

[mucour]

Okay CHD01, if the edition you have gives a correct number, kb=0.89, then the problem is with the edition I have, though it is of a higher edition than yours.

Thanks for your time.