Relief Line Sizing

[homerphish]

Does anyone know the proper formula to use to determine the pressure drops along a relief piping line? IIAR seems to have a couple of non-descriptive methods.

 

[imok2]

homerphish, Please be a little more specific and maybe we can help you!

 

[sterl]

The ASHRAE 15 addendums of 2000, numbers C & D , Appendix H shows the math that was endorsed for use for refrigerants and refrigeration type SRV's of the Pop Off- 25% +/- Blowdown, Direct Acting Types....The most common variety. Appendices tabulate this though a number of engineering firms have programmed the Network Math for multiple valves discharging at different set pressures to a common header.

There are other devices and alternative math...See the API 520 and articles in Semple, Anderson Greenwood and Crosby (manufacturers, Anderson Greenwood published in 1989, Chemical Engineering or proceedings of the AIChE though I don't have the specific source at this moment) and detailed what was the basis of concern for Petrochem and Air Cracking plants in tuilizing similar deep-blowdwon valves...

The authority on the evolution of the ASHRAE 15 to incorporate this approach was Mr. Wm V. Ricahrds of Illinois, consultant...

Follow up work by Mr. D. Kuespert of Snowy Owl LLC and others...Also consultant.

 

[imok2]

homerphish
VALVE SIZING AND SELECTION
Step 1: Use the formula below, per ANSI/ASHRAE 15-
1994, "Safety Code for Mechanical Refrigeration" to
calculate the minimum required discharge capacity in
pounds of air per minute. When selecting a dual pressure-
relief valve system, be aware that each individual valve
must have sufficient capacity to protect the vessel.
C=fDL
C = minimum required discharge capacity of the relief
device in pounds of air per minute.
f = factor for ammonia refrigerant is 0.5**, factor
for R-22 and R-134a refrigerants is 1.6**. for
other refrigerants see ANSI/ASHRAE 15 or
contact factory.
D = outside diameter of vessel in feet.
L = length of vessel in feet.
** This factor is not suitable when combustible materials are
within 20 feet (6.1 m) of the pressure vessel; refer to relevant
codes for corrected sizing method.
Example: To determine the minimum required capacity
of a relief valve for a vessel containing ammonia that
measures 16 feet in length and 6 feet in outside diameter,
the equation would be as follows: 0.5 x 6 x 16 = 48 lb/
min
Step 2: Determine the pressure setting needed. This
should be at or below the design pressure of the vessel.
The relief setting should also be at least 25% above the
maximum expected operating pressure to avoid "weeping"
of relief valves. The setting may be below (never above)
the design pressure of the vessel, but it is sometimes
best to match vessel design pressure and relief setting
to minimize the likelihood of a discharge.
Step 3: Refer to the Valve Capacity Ratings below and
select the valve with the required capacity (C) at the
desired pressure setting.

 

[imok2]

You can also go here:

http://www.hantech.com/Literature/PDF/k109e.pdf

 

[homerphish]

I know how to calculate the pressure drops and size the valves, what I am uncertain of is the back pressure. I have found the maximum allowable back pressure of the valve by taking 15% of the set pressure and adding atmos pressure. This value represents the maximum allowable pressure drop along the line. When there are mutiple valves with different set points into the same header, the pressure in the header is to be assumed as the value of the lowest valve set point. Is this correct?

 

[NH3Valves]

For multiple valves at different setpoints discharging to a common header the process for sizing is somewhat complicated. IIAR Piping Handbook (2000) suggests 15% allowable backpressure per ASHRAE 15-2000 as opposed to ASHRAE 15-1994 or earlier that allowed 27.5%. The piping handbook offers all relative equations and examples of how to perform the calculations. The procedure is as follows.

Find the Kx value for the lowest setpoint valve
Kx=.2148(Po^2-P2^2)/f example: Kx for a 150 psig setpoint valve with sch. 40 pipe is 12,530

Kx is the available resource for all of the relief piping.
The sum of the (L*Cr^2)/d^5 for all of the relief lines as well as the header must not exceed this number. So one needs to pick out diameters for the individual relief piping and common header until this is satisfied. Hope this helps.

 

[NH3Valves]

It appears I left out a few important details from my previous post.

Po=.15*set pressure + atmospheric pressure
P2 is the outlet pressure of the relief piping (usually atmospheric unless relieving to the low side)
f=.02 for sch. 40 black pipe
L is length
Cr is the rated capacity of the valve in lbs/min
d is the ID of the pipe

I highly recommend using the piping handbook as a reference. Also, you can call your relief valve manufacturer. I work for one and am always willing to assist. jjnelson@parker.com

 

[sterl]

I'm not sure that homer's question is addressed:

The pressure in a common discharge header needs to be no higher than the lowest set point valve's setting, times .15, with ALL VALVES THAT COULD COINCIDENTALLY OPEN TO THAT HEADER, AT FULL RATED FLOW...with the exeptions that vessels that could not reasonably encounter the same (fire or heat exchange) conditions, can be treated as never discharging coincidentally.

A collection of PSM consultants have devised the software and there have been a number of spcific solutions devised for systems that did not have a large enough header, but most discharge installations prior to the 2000 addendum will not conform to the 2000 requirements...it is a substantive revision.