A search for a better material of construction should be weighed against the cost and long lead times of exotic materials, the learning curve of maintenance with new materials and techniques, compatibility issues, and cost of retrofitting. Sometimes, that cheap steel bolt that must be replaced in six months is a better choice than one of an expensive hard-to-find nickel alloy with an unpredictable reliability history. This same logic can be applied to cladding, which is another appealing option.
Dirk Willard, consultant
Our refinery, which is located in the upper Midwest, runs into problems pumping liquefied petroleum gas (LPG) from storage bullets to tanker trucks during the summer. Except for low tank level, we can pump 600 gpm in the winter without any issues but have trouble pumping at all by late morning in the summer. That's when we divert propane, with some difficulty, to underground pipelines.
The bullet pressure reliefs are set at about 500 psig. The operating pressure starts at about 140 psig when the bullets are full but is only about 105 psig at about 45% full, which is as low as we can go. We feed four bullets at a time to the pump. During filling, pressure can spike at above 200 psig on a hot day.
Suction piping: the bullets have a single 4-in. discharge through a ball valve; flow is through about 150 ft of aboveground 8-in./10-in pipe. Discharge piping: two dryer tanks separate liquids from the LPG; there also are two filters and a strainer. (We haven't kept maintenance records for the dryers or filters.) A 600-ft aboveground run of 6-in. pipe goes from the 8-in. pump discharge to the flow control valve; a 2-in. hose connects the skid to the truck.
Typical loading of the rack, with 600 gpm, is at about 225 psig, immediately upstream of a 4-in. equal-percentage globe valve (CV = 220) with a final pressure of about 150 psig at the tanker truck. A turbine meter immediately upstream of the valve measures flow erratically. During startup, the flow is 100 gpm at 30 psig, with a pressure of 270 psig into the valve.
We need to address vapor lock of the valve during startup and pump cavitation during low level. We're seeing a one-year life on the seals in these multistage inline vertical LPG pumps. During the summer we always have trouble starting a pump once it has stopped. A manual vent line at the pump bowl connected to a flare is used to bleed off the gas. We always see a surge when we start the pump, hot or cold. There are secondary 2-in. lines at the top of each bullet for manually venting them to flare.
What is the cause of our pump problems? Would using a smaller valve help? What can we do to improve this operation?
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