Bearing pad size under 24" valve....

[hp48sx]

Good evening,

I have a question about an application of a bearing pad. We have an application where a 24” ball valve in a pump station is mounted to a concrete pedestal. The ball valve is located “in-line” between a large pump and a pipe header distribution system. We are suppose to use a bearing pad and limit the compressive stress on the pad to no more than 12 psi.

Based on the Dead load of the valve assembly and the some contributory load of the surrounding appurtenances (15,100 lbs) I have calculated a required minimum baseplate and pad size of 36” x 36” square (11.65 psi).

The engineer is now telling me that there is a startup pressure on the closed valve of 61 psi – this pressure is relieved as the valve is opened and the pump continues to run. He says that an overturning moment is induced in the valve base support and thus creates an additional compressive force on the bearing pad. With a calculated approximate force of 27,500 lbs on the closed valve (61 psi x area of a 24” dia pipe) and a moment arm from the top of the support to the centerline of the load equal to 29.5 inches, the design “blows up” and a bearing pad could never work.

It seems to me that any horizontal force applied to the closed valve would first go into the surrounding pipe and pump supports before it is transmitted through the valve bearing pad and into the valve support.

What guidance can you give me to help me limit the applied stress in the bearing pad to 12 psi?

Is it even reasonable to limit the stress in the pad to 12 psi in such an application?

Thanks in advance for your help.

 

[bimr]

Where did you come up with the 12 psi? The bearing stress is something that you obtain from the valve manufacturer.

Here is an example, where the valve manufacturer is calling out 3000 psig:

http://www.henrypratt.com/products/specs.asp?ID=21&Name=Cone+Valve&S=y&D=n&L=y&P=

The maximum loading is not at startup or when the valve is opened. The maximum loading would probably be when the pump is operating and a power failure occurs causing a quick shutdown.

In other words, the maximum forces are caused when you have changes abrupt changes in flow. To minimize these maximum forces, the engineer will use slow closing valves, spring loaded check valves, etc.

The pressure in the pipe is not going to increase the pressure on the bearing pad.

 

[bimr]

http://virtualpipeline.spaces.live.com/blog/cns!EE3CE5E6AEE6F452!609.entry

 

[hp48sx]

The 12 psi was given to me by the "approving" engineer. It sounds way to low of a bearing pressure to me.

 

[bimr]

What do you mean by "approving" engineer?

 

[hp48sx]

In this application the engineer of record has specified that the installing contractor submit calculations on the valve base plate demonstrating that the bearing stress from the baseplate to the bearing pad is less than 12 psi. They cannot justify why they selected 12 psi, even after I submitted an RFI requesting confirmation that the bearing pressure was 12 psi and not 120 psi or 1200 psi.

 

[cvg]

how is this overturning moment created? ask the eor to draw you a free body diagram and demonstrate this to you. Seriously, the force is horizontal and unless the valve is not free to slide on the pedestal, than I have a hard time seeing how a moment is created which would cause pressure on the foundation.

36" square seems a bit large to me. 15,000 pounds is a lot - perhaps you need some intermediate supports on the piping also? Maybe he is interested in keeping the pressure on the foundation below 2,000 psf?

 

[rconner]

When you say "bearing pad", are you talking about a relatively thin layer of non-metallic or concrete something (like rubber or kraft paper etc.) somehow sandwiched between the pier or pedastal structure and the base of the valve? If so or not, was curious what is the exact pad material or anchorage that causes the Engineer apparently to be worried about stresses in excess of 12 psi?

 

[hp48sx]

The bearing pad is a 1/2" elastomeric pad intended to isolate the valve base from the vibrations in the the piping system. I'm not sure what the justification is for the 12 psi. The manufacture's data sheet says that the "lowest" listed pressure capacity is 50 psi.

 

[jte]

hp48sx-

With respect to the longitudinal force - moment issue, I would respectfully tell your engineer to go back to school. Simply put, if the valve was an end cap, where would the end load go? It is handled by the longitudinal stress in the pipe. None, yes, none of it goes into the pad. This is easy enough to test. Take a household garden hose. Put a spray nozzle on it. Turn on the water. Close the spray nozzle. Set it down. Does the end load on the closed spray nozzle do anything? Just because you have a system about 36 times larger doesn't change the way the reaction works.

Actually, simply deriving the longitudinal stress from a free body diagram proves this: The load due to pressure times area which you've already calculated is carried by the cross sectional area of the pipe, or pi*2*r*t. So force over area is P*pi*r*r/pi*2*r*t = P*r/2*t. Look familiar? (The hoop stress formula is derived in the same way, just take a unit length of pipe, cut it in half and look at the force/area.)

By the way, you would get a whole lot more exposure to piping engineers at forum378: Piping & fluid mechanics engineering.

jt

 

[vzeos]

The 12 psi sounds like it might be the minimum soil bearing capacity.