Take the Pressure Off Loading

Readers suggest how to avoid risks during upsets.

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C.C.S. Reddy, lead process design engineer
Singapore Refining Co., Singapore


INSTALL RESTRICTIVE ORIFICES
I suspect that the pump is being operated by a variable frequency drive. I assume the pump is a centrifugal radial type. Higher speeds are probably possible — but why would you want to go there? Pump wear is to a power factor of three. Going above 3,600 rpm increases wear on bearings and seals. Perhaps this pump can run at a higher speed but this approach should be discouraged. Instead, install an orifice plate or a series of plates for extreme pressure. A restrictive orifice (RO) will bring the pump back on its curve during unloading. Size the RO ΔP using the difference between the extreme pressure and the design pressure. Install the RO upstream of the relief valve to prevent over-pressurization of the piping.

Think about the possibility of other equipment damage. Inspect the pump case and all equipment upstream of pump discharge to assure they will survive the higher pressure. It helps to remember that pressure ratings are based on the probability of failure. A valve rated for 150 ANSI may survive 200 psi without a leak but the odds of leaking or even bursting increase exponentially above its rating. If the expected pressure exceeds ratings, why not install a relief valve upstream of the pump in the suction piping?

Will the pump act as a brake on the system? I doubt it. The shaft will turn counterclockwise. The impeller may eventually spin off. Check to see if the impeller is keyed in such a way to prevent this. Turning the shaft will not be detrimental to the motor, although there could be a risk to turning a motor into a generator.

Dirk Willard, senior process engineer
International Steel Services, Inc., New Caledonia



MAY'S PUZZLER
We had six railcars in a propylene loading bay (Figure 1). One of the tankers was over-filled and was vented to fixed piping via hose at the loading station; the fill valves on the car and the loading arm were open. The other cars' valves and their loading valves on the fixed piping were closed per standard procedure. Sometime during the night two additional cars were brought down the line to be filled, pushing the first six cars towards the exit and blocking the entrance track point. The locomotive crew -- believing the first six railcars were full and disconnected -- bumped the cars further down to access other bays. In the process, a hose snapped. The emergency cut-off valve for the bay failed. The only way to prevent a major catastrophe was for an operator to rush in to manually close the valve. He stopped the flow but received serious cold burns. Fortunately, the propylene vapor cloud didn't ignite. How can we ensure that nothing like this ever happens again?

Send us your comments, suggestions or solutions for this question by April 9, 2010. We'll include as many of them as possible in the May 2010 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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