Relief Valve Sizing

[GBRD]

G'day Everyone,

Can anyone guide me through the process of selection of relief valves & its piping? Looking for a ammonia system that includes 2 stage refrigeration.

regards

[Dacosta]

The first step in sizing a relief valve is to locate a copy of the Uniform Mechanical Code (UMC) (or whatever code book you follow, (the California Mechanical Code for me)) (or ASHRAE 15).

The next step is to collect the basic information about the vessel you are trying to protect. Namely the diameter and length of the vessel, as well as the pressure rating. While the diameter and length of the vessel is straight forward the pressure rating can be a problem. Modern vessels should have a name plate with the make, model and pressure rating on them. But over the years the name plate can be painted over, scratched up, knocked off, covered with insulation, covered with ice, or never had one to begin with (very old vessels). Consider replacing any vessel without a pressure rating.

The pressure rating of the relief valve should be less than or equal to the pressure rating on the vessel.

The capacity of the relief valve is calculated by the following equation.

C = f D L

Where
C = is the minimum required discharge capacity of the relief device.
f = factor dependent upon type of refrigerant
D = diameter of the vessel
L = length of the vessel

Use your code book to determine what value of “f” you should be using.

The next step is to select a relief valve based on the minimum discharge capacity calculated at the pressure rating. Try and find a relief valve that is the closest match to the required capacity without going bellow this value. To large of a valve will cost you in piping in the next section.

It is common practice to pipe all the relief valves into a common header and have the common header discharge at a single point, or into a diffusion tank for ammonia. Therefore estimate the total length of each branch header plus the common header lengths for each relief valve.

Calculate the Allowable Equivalent length of discharge piping. Using the following equation

L = .2146 d^5 (Po^2-P2^2)/ f Cr^2 - d ln(Po/P2) / 6f

Where
L = Allowable equivalent length of discharge piping
Cr = rated capacity as stamped on the relief device.
f = Moody friction factor in turbulent flow
d = inside diameter of pipe
ln = natural logarithm
P2 = Absolute pressure at outlet of discharge piping
Po = allowed back pressure at the outlet of pressure relief device.

For conventional relief valves 15% of set pressure, Po = (.15 P) + Atmospheric pressure

(there is also a separate equation for using metric units)

The Moody friction factors are listed in the appendix of the ASHRAE 15 or the IIAR Ammonia Refrigeration Piping Handbook.

Increase “d” until the allowable equivalent length is longer than the estimated total length of the branch headers and common headers. Typically the “d” will be one or more pipe sizes larger than the outlet of the relief valve.

Sum total all the cross sectional area of the individual branch headers. Size the common header based on cross sectional area larger than the sum total of the individual branch headers.

For example if 3 – 1 in diameter pipe, each pipe has a cross sectional area of .719in2. The sum total is 2.157in2. The main header should be a 2 ½” with a cross sectional area of 2.469in2. (2.469>2.157)

There are a few more requirements listed in the UMC but this should get you on the right track. The Ammonia Refrigeration Piping Handbook has a slightly different method called the “Resource Method,” its more flexible with complicated relief piping scenarios, And may reduce your pipe sizes in certain locations

[GBRD]

Its simply an elaborated well explained reply Dacosta. Great work. Thanks mate.

regards

[TXiceman]

This will work if the customer accepts ANSI 15 standard calcs for a refrig system. Some may require you to use thwe longer ASME calculation method and fire sizing criteria.

Ken

[GBRD]

How does that work Ken? Its always good to know that..

regards

[TXiceman]

Check with an industrial PSV manufacturer like Consolidated and they lave published the API or ASME PSV sizing procedures. For most A/C and refrigeration projects, the ANSI 15 calcs will be sufficient. Where you will run into the ASME or API requirements is in chemical plants and refineries.

Ken

[Magoo]

The ASME codes apply for vessels in Australia an NZ.
magoo