PG-71.5

[loilfan]

PG-71.5 of ASME I says:
"When a boiler is fitted with two or more pressure relief valves on one connection, this connection to the boiler shall have a cross-sectional area not less than the combined areas of inlet connections of all the pressure relief valves with which it connects and shall also meet the requirements of PG-71.3"

PG-71.3 also says:
"The opening or connection between the boiler and the pressure relief valve shall have at least the area of the valve inlet".

What do they define as the combined area of the PSVs? Only nominal pipe size, or do we consider the ID/OD instead?

Let's say I have two identical 1.5x2" PSVs attached by a 2.5" nozzle on a boiler. Using the NPS areas, this means that the combined areas of inlet connections is 2*PI/4*1.5^2 = 1.125PI in^2 and the nozzle connection is PI/4*2.5^2 = 1.5625PIin^2 and is acceptable.

 

[Latexman]

The ID is the only one that makes sense to me.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[loilfan]

This requirement is still odd to me. Let me explain why:

In my example above, let's assume I have Sch 80 pipes and that my nozzle connection is now 2" instead of 2.5". The 1.5" pipes attached to each PSV has a combined area of inlet connections of 2*PI/4*1.5^2 = 1.125PIin^2. The nozzle connection cross-sectional area is PI/4*1.939^2 = 0.940PIin^2. This would not be acceptable.

However, if I then changed my PSV inlet pipes from Sch 80 to Sch XXH, I now have a combined area of inlet connections of 2*PI/4*1.10^2 = 0.605PIin^2. This is less than the 2" Sch 80 connection and is therefore acceptable.

So my configuration only becomes acceptable if I decrease flow to the PSV?! That can't be right.

No one's using the orifice area for this calculation, are they?

 

[Latexman]

What's the set pressure?


Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[don1980]

This is a fairly common question, since ASME code (Sec I and Sec VIII) doesn't explain whether it is referring to nominal area or true x-sec area. ASME Sec I addressed this question in a published interpretation. See the attached image. Personally, I'm surprised that ASME interpreted it this way.

 

[loilfan]

LatexMan, 150 psig. What does the set pressure have to do with the inlet required area?

don1980, thank you for the interpretation reference. Unfortunately I don't see how to use that. The way I understand it is that the valve inlet area is the diameter where Pv is written in Figure 8 of API 520. i.e. it's the bore diameter of the inlet flange. I attached an image for your reference.

This makes sense to me, however, I have yet to see a manufacturer list this dimension. Using the orifice area would be incorrect because some nozzles have internal reductions in diameter.

 

[don1980]

It's simply saying that all of the inlet piping elements must have an ID that is at least that of the actual ID of the PSV inlet flange. For example, if the PSV inlet flange is 2" 150 class, then you'd technically be violating this requirement if you had a sch 80 segment of inlet pipe that had an ID less than that of the 2" PSV flange.

BTW, PSV flanges are standard ANSI flanges, so you can easily get the ID without referring to the vendor's drawings.

 

[Latexman]

I usually work with Sec. VIII, which is similar to Sec. I. Pressure drop in the PSV inlet is usually limited to 3% of conventional PSV set pressure. I was trying to get a feel for the amount of pressure drop that was available. The cross-sectional area of the pipe and fittings that make up the inlet to the PSV has a strong influence on the pressure drop.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[loilfan]

don1980, B16.5 does list the minimum Bore for flanges. There are three columns (Slip-On, Lapped, and WN/Socket) all with different values.

We can't use the columns for Slip-On or Lapped Flanges because their bores are larger than the OD of the pipe.

Note 7 of Table II-8 (150# Flanges) says "Dimensions in column 13 correspond to the inside diameters of pipe as given in ASME B36.10M for standard wall pipe.....These bore sizes are furnished unless otherwise specked by the purchaser". It is possible for a flange to be bored to a different diameter than Sch STD. Are we supposed to assume that all PSV manufacturers use Sch STD bores unless their catalogues indicate otherwise?

 

[don1980]

I forget that we're talking about Sec I boiler PSVs, and not Sec VIII API 526 valves. API 526 PSVs are fabricated with weldneck flanges, but that not necessarily true for a Sec I boiler PSVs. So, I suggest you refer to the vendor catalog for that dimension.

 

[loilfan]

I only have the 2002, 5th edition of API 526 but I don't see it mentioning weld necks. Was it added to the latest edition?

For ASME VIII-1 PSVs, does that mean that any inlet piping greater than Sch STD with the same NPS would be a restriction?