5 replies to this topic

[robjul]

Hello,
I witnessed an ongoing construction where Diesel is pump out from its tank and discharge into a Gantry. The centrifugal pump discharge pressure is 230 psig (I was not told of the suction pressure). I have learned that the piping class installation was for 150psi. The contractor said that it is ok since the allowable working pressure for piping is several thousands psi, to my surprised?! Is this true/ok? Appreciate your comments. Thanks.

[ankur2061]

robjul,

The maximum allowable working pressure for the 150# rated piping would be a function of the maximum temperature of the fluid being pumped. For any piping system the flange is the weakest element in the piping and thus governs the MAWP of the piping . ASME B16.5 - "Pipe Flanges and Flanged Fittings" provides the MAWP or Design pressure for any class of material (CS / SS / Alloy) and any pound rating based on the temperature of the fluid.For example, if CS is the material and 150# is the rating then for a temperature of 40 deg C the MAWP for the entire piping system would be 284 psig (19.59 barg).

There is a nice on-line calculator based on ASME B16.5 for finding out the MAWP at a selected temperature for a given pound rating (150, 300, 600, 900, 1500, 2500) and material of construction (CS / SS / Alloy Steel) at the following link:

http://www.gowelding.com/pv/flange.htm

Hope this helps.

Regards,
Ankur.

[robjul]

Thanks very much Ankur! you're an angel. For a while I thought that if the rating is 150#, the pipe must not be operated above 150#.

[fallah]

I have learned that the piping class installation was for 150psi. The contractor said that it is ok since the allowable working pressure for piping is several thousands psi, to my surprised?! Is this true/ok?

robjul,

The contractor certainly means maximum allowable stress instead maximum allowable pressure. Is it right? Please check and report.

Fallah

[robjul]

Hi Fallah,

To be honest, I also got confused on what really did the contractor said; -- if it's maximum allowable stress or maximum allowable pressure? But I also believe that the contractor means max. allowable stress. On the other hand, I was also thinking if there is a sensible logic on what the contractor said. That since the pressure is the one primarily contributing to stress (subject to operating temperature), no matter how much the pressure increases beyond the 150# pressure rating, still it is very far from the allowable stress values?! Looks like the difference between allowable stress and allowable pressure was not fully comprehended by both of us considering these are fundamental concepts?
Sometimes, simple concepts invites confusion like in the proper use of gauge and absolute pressures. I had been dealing with this for decades and still find myself up in the air sometimes, committing the error of using gauge rather than absolute pressure. I can't help but consider this simple basic learning, a big shame. Any advice on how to get away from this trap? Indeed, the more you know, the more you do not know!?

[fallah]

Looks like the difference between allowable stress and allowable pressure was not fully comprehe
nded by both of us considering these are fundamental concepts?

robjul,

I have no problem for comprehending the concept, you? i don't know:

Pressure is the force per unit area applied in vertical direction to the surface of an object.

Stress is the internal forces acting within a deformable body and is a measure of the average force per unit area of a surface within the body on which internal forces act.

The dimention of the stress is that of pressure and obviously looking at a pipe, especially large size, with internal pressure one can sense allowable stress (acting on cross section of the pipe shell that has low area with respect to total cross section of the pipe) would quantitatively be much higher than allowable working pressure.

Fallah