Pumping LPG: Take A Load Off Loading Readers suggest how to avoid problems in pumping LPG
THIS MONTH'S PUZZLER
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?
CHANGE THE VENT
The tank pressure varies with the ambient temperature, 140 psig in winter when ambient temperature is lower and up to 200 psig in summer. It also appears that the tank vapors are not vented back to either the process or the trucks. Therefore, when filling with liquid, the rising level has to compress the vapors until they can condense and this takes time. Keep in mind that the liquid propane is always at bubble point in the tanks. Any heating or pressure drop will cause flashing. Also, many of these tanks make-up and pump-out from the same end, resulting in stagnant product. Putting the make-up into the tanks at the opposite end and in the vapor space is better.
Then, there are the pump issues. The pump does not have enough net positive suction head available (NPSHA) to run properly. When the winter tank level gets down to low level, there isn't enough head to feed the pump and the suction vaporizes and cavitates in the pump. The pump is probably always cavitating as evidenced by the erratic flow in the turbine meter. Suction strainers are notorious for causing cavitation. You should have a 2-in. line from the pump discharge (before the check valve) back to the tank vapor space. When starting the pump, or when it's vapor-locked, open the line and recycle the LPG back to the tank — this can be automated or manual. Each pump will require a startup line.
The pump suction line is also important. Typically, the suction line should be at least 12-in. — one size larger than the suction flange — right up to a reducer on the pump suction flange. The 4-in. discharge valves on the tanks must be full port and should increase to at least 6-in. immediately after the valve. Everything you can do to reduce pressure drop will help. Also, the piping must not have any pockets that would trap vapors. Furthermore, the suction pipe is long; move the pump closer to the bullets.
Consider the pump discharge. The 6-in. discharge line looks okay, 0.5 psi/100 ft and 7 ft/sec velocity. The rack control valve maintains pressure so that the turbine meter can operate properly. Size is not critical. If the valve is open less than 30%, consider installing a reduced trim in the valve. When the pump stops, the LPG trapped in the discharge line should be vented back to the bullets. A small valve and tubing back to the vapor return line will ensure that some of the LPG is always returned to the bullets and will keep the line cool when flow is low.
So, what's missing? A vapor return line. A truck is required to have a vapor equalization line that connects back to a common header on top of the bullets. The header also allows the tanks to maintain an even level.
Climate is also important. In winter, the tanks are about 83°F at 140 psig, and in summer about 108°F at 200 psig. In winter, the ambient temperature is usually colder than 83°F and so Mother Nature is helping cool the LPG in the suction line and thereby adding NPSH. However, in summer, the sunlight on the tanks and piping can heat the LPG enough to cause vaporization. It is best to keep the suction piping in the shade provided by the tanks. Also try painting the lines white or consider insulating the lines, but be careful about corrosion under insulation. Do not assume this will solve all of the problems. You need vents on all high points and any length of horizontal piping with the vent valve piped or tubed back to the bullet vapor space.
Larry Tarkington, project engineer
Company undisclosed, San Antonio, Texas
MAINTAIN THE BULLET PRESSURE