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RE: Pipe Support Forces for Transient Flow

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Bill,
 
Most valves shut slowly, reducing the "water hammer" effect that you see on typical sink valves. We control the pressure wave by using timed valve closing rates.
 
You can use a percentage of piping MAOP if you can not control the valve closing rate.
 
Most piping systems have slugging forces that depend on the fluids that result in high forces and can lift the pipe from the supports. Evaluate slugging forces on a project basis and control movement with directional guides and anchors.
 
Bill
-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Thursday, February 16, 2006 10:40 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Pipe Support Forces for Transient Flow

Since Rich Lewis brought up pipe support design, I have other questions related to design forces on pipe supports. 
 
It seems to me that a piping system with fully restrained pipe joints should not have any unbalanced external forces due to either internal pressure or flow, when under steady state flow conditions (except for vertical weight).  However, when rate of flow changes, such as due to valve closure, there will be a pressure wave in the pipe that will cause unbalanced forces at pipe bends - such as "water hammer" effects.  For major piping systems, detailed transient flow analyses can be performed using computer software.  But for many piping systems, such transient analyses are not performed. 
 
So the question is, if a detailed transient analysis has not been performed, do any standards or guidelines define a minimum design force to be applied to an anchor for a straight pipe run between two bends?  If the piping only undergoes thermal changes due to ambient temperature changes, there would only be minimal axial force due to friction of the supported piping.  But it seems to me that there should be some minimum recommended design force for the anchor to handle the unknown transient flow forces. 
 
The book "Piping Handbook" (Edited by: Nayyar, Mohinder L., 2000, McGraw-Hill) provides some useful discussion of flow transients and recommends that transient conditions be considered in pipe support systems.  But it stops short of recommending design forces where a transient flow analysis has not been performed.  So far, I have not found a good source of information for such design.  What do engineers who regularly do pipe support design do when given a piping system to design for but without unbalanced forces defined?
 
(Rich, I recently reviewed a piping system very similar to your arrangement, except that it did not include expansion loops.  The piping was shown to be supported from below on a steel frame - the pipe-to-frame support details were still being worked out.  I proposed a single anchor point near the middle of the straight run, but the above question came up about what force to design the anchor for.)
 
William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 


From: Rich Lewis [mailto:seaint03(--nospam--at)lewisengineering.com]
Sent: Wednesday, February 15, 2006 3:45 PM
To: seaint(--nospam--at)seaint.org
Subject: Pipe Support Detail

I read some about this subject in the archives but couldn't find the answers to my questions.  I have a situation where I need to support two 12" and one 14" diameter horizontal runs of water lines about 20 feet above the floor.  I am not able to hang them from the roof structure so I need to design a frame to support them.  I will have anchor frames at each end and an expansion loop in the middle.  Right now I am looking at the typical intermediate frame supporting the pipes about 23 feet on center, not the anchor frames.  I was considering a Tee type frame or a vertical stick frame with outriggers to support the pipe.  I prefer the Tee system because I could more or less balance the loads across the top.  I was told if I could stack the pipes vertically instead of horizontally side by side it would help with the expansion loop.  Using a stick type frame with outriggers puts all the load eccentric to the column on the same side.

 

I think what will influence my final decision will be the type of connectors of the pipe to the frame.  There is no insulation on the pipes.  As I see it I need some type of slide or roller connection to allow the pipe to expand and not push the frame.  I looked at bare steel friction sliding and decided it was too great to fight with the frame.  I looked up a sliding type pipe connection on the internet to set the pipe on the top cross member of the Tee.  I was thinking that maybe if I hung them from the beam instead of sitting on top I could have a swivel hanger type connection.  If I use one of these though I would think I need a swivel at the pipe bracket and at the beam hanger nut location.  I didn't see any like this.  As I see it, this would also put a slight rotational moment around the support column.  I saw there are roller type swivel connections.  This would take out the horizontal component of the pure swivel.  Unfortunately there load carrying capacity was lower than I need of about 3,500 pounds.

 

I'm looking for suggestions in two areas from engineers who have experience in pipe support:

  1. Is it better to have the supports sitting on a cross beam or hung form a cross beam?
  2. What connection material would be recommended to allow the pipe to move longitudinally relative to the support frame?

 

Thanks for any insight.

 

Rich