High Point in Piping Downstream of Pump

[cbnorthside]

We have a positive displacement pump transferring crude from an existing storage tank to a new tank. The pump discharge piping is at an elevation of 33 meters. The piping has a high point section at 72 meters and then drops back to an elevation of 56 meters entering the new tank. There are two flow meters: one on the pump discharge line and one on the line entering the new tank.

When the pump is shut down, crude will drain from the high point section to the new tank, creating a potential vapor space in the high point section. A potential concern is running slugs of vapor through the downstream turbine meter.

The recommended solution from our client is to add a backpressure regulator on the line as it enters the new tank and set it at a pressure high enough to prevent vapor formation in the line (30 psig). There was an additional recommendation to add a small recirculation line with globe valve on the pump discharge back to the existing tank. When the pump is to be shutdown, the operator would open the recirculation globe. The pressure regulator would slowly close to maintain the required line pressure. The globe valve was seen as a means of smoothing out pump shutdowns and pressure control.

Has anyone encountered a similar operational / startup problem like this one and does the recommended solution feasible? Or are there other better options worth considering? I was not sure the 2nd recommendation was really necessary once the regulator was added.

Any feedback would be appreciated.

 

[JJPellin]

We have had some issues with sewer pumps in a similar situation. As you are aware, you will recover the head needed to push fluid up to the high point as the flow "siphons" down the other side. But you can only recover about 32 feet of head since you will draw a vacuum and vaporize product at the high point. We ended up instaling a flow orifice at the far end of the line to keep from continually forming a vapor pocket at the high point. This did not prevent the line from draining down partially once the pump was shut off. A back pressure regulator would be better and could hold pressure even at shutoff. The reason for the spill-back recommendation is probably just to avoid a water hammer when the pump is restarted. If the check valve at the pump leaks at all, the leg of liquid from the pump up to the high point will still drain back. The vapor pocket that forms at the top will collapse quickly when the pump is restarted and the resultant water hammer could damage the downstream system. Since the vapor pocket is composed entirely of vaporized product, once pressure is restored, it should re-liquify and there should be no gas left to mess up the downstream flow meter.

 

[stanier]

A flow control valve to provide the headloss to lift the hydraulic grade line above the elevation is common practice.

Bypass around the pump could be preferably a relief valve so you dont rely upon the operator to open a valve.