4 replies to this topic

[chem55]

I have an application to "Top offload" viscous chemical (1600cP) from the Rail car and the Gear pump is located at the Grade. The capacity of the Gear Pump is 150 GPM. Suction line is 4" and discharge line is 3".

The elevation of the bottom of the rail car and highest point of the loading arm is 20ft. Is it possible to have gear Pump which unload the material without pressurizing the Rail car with Air/Nitrogen? I am planning to pressurize the rail car with air to lift the material and transfer to the pump.

Does anyone have experience with this kind of system?
-Is there any issue with trapped Air in the suction line?

Attached Files

•  RAIL car top unloading sketch.xls   67.5KB   109 downloads

[Art Montemayor]

Chem55:

I have used this system – with pressurization of nitrogen in the railcar.

The main importance of the loading arm’s elevation is that it represents the length of the suction line leading to the gear pump and, thus, is a factor in calculating the NPSHa.

I had to pressurize the railcar to assist in increasing the NPSHa to the pump. You must first calculate the proposed NPSHa to your pump and compare it with your pump’s required NPSH (NPSHr). Normally you should allow for an excess of 1.5 to 3 meters (as a minimum) of your NPSHa over that of the pump’s NPSHr. Of course, this is dependent on the fluid being pumped (its viscosity) and the pump’s age (internal clearances, efficiency). Although your suction line size seems appropriate for a liquid with a viscosity of around 1-25 cP, you should calculate your pressure drop through the suction line for the viscous 1600 cP fluid and figure your NPSHa based on that. Don’t worry about the height of the loading arm – the excess height over the lowest liquid level in the railcar is all “recoverable” head. Your worse suction head is the difference between the lowest liquid level in the railcar and the center of the suction inlet to the pump. If you find that your NPSHa is too close to (or lower than) the pump’s NPSHr using the railcar vented to the atmosphere, then you can resort to applying a gas pressure to the railcar to increase the NPSHa.

Before attempting to start the pump, you must have it PRIMED. All pumps have to be primed before startup. Otherwise, they can’t work. Even so-called “self-priming” pumps have to be primed. In order to prime the pump, you must fill the entire length of the suction piping system with 100% liquid – preferably the same fluid being pumped. You can do this with a hand pump or using gas pressure in the railcar to fill the suction line all the way through the pump’s casing and preferably a little beyond its discharge outlet. The most efficient way is to use gas pressure inside the railcar because the foot valve (check valve) used in the suction dip pipe in the rail car (which you failed to show) often leaks and gives problems in trying to “back fill” the line through a nozzle down stream of loading arm. Some people even used a partial vacuum to draw fluid through the suction line and through the pump casing. There are various ways of priming a pump.

If you prime the suction line properly, there should be no "trapped air" in that section of line. If you use a pressurized railcar, you may get gas there, but when you stop and shut down the system, it should drain down the line.

I hope this answers your questions.

[chem55]

Thanks for your valuable inputs Mr. Montemayor.

Please correct me if my understanding is incorrect:
I am planning to pressurize the Rail car so I think there is no issue with NPSH in this case as there is always 20’ head available at the suction side, and due to that my NPSHa is well over 40ft even when the fluid in the Rail car is at the low level – also in my calculation I am pressurizing the Rail car to 12 psig to avoid vacuum at the high point in the suction line (considering the fluid is at the low level) and after that fluid should fall under the gravity to the pump suction. The suction and discharge sizes are restricted so I couldn’t change it.

As I am pressurizing the Rail car, is there any issue with “trapped air” during the startup? I think there will be some air in the suction line when I connect the loading arm to the rail car and start the operation, which may create the operational issue as either the air will get trapped at the suction high point OR air bubble will pass through the pump.

[Art Montemayor]

Chem55:

Let’s go through the procedures of pumping out your railcar, in order to get everything well understood:

1. Start out with the piping and hose connections already made and in-place. All connections are tested and not leaking. The unloading suction line to the unloading pump is initially full of air – as well as the pump casing itself and the discharge piping all the way through to the discharge block valve.
2. You should have a vent valve located at the highest point of the unloading arm (or line); you should also have a vent valve located at a point immediately before the pump’s blocked discharge.
3. You admit enough pressurized nitrogen (or air) into the vapor section of the railcar to elevate the contained fluid over the highest point in the loading arm (or line). You don’t want more than this because you don’t want to waste fluid that will be vented through the vent valve in the loading arm. You shut the loading arm vent valve as soon as fluid starts to come out through there.
4. You either increase the nitrogen pressure to allow the fluid to flow through the pump and out through the discharge vent valve OR you simply force fluid through the pump by setting the nitrogen vapor space pressure to the pre-determined level and proceed to start the pump. The pump will kick-in and start pumping fluid with nitrogen in front of it out of the discharge vent valve. You quickly open the discharge valve and shut the discharge vent.
5. Your pump is on line and you are pumping 100% fluid into your reception system (presumably a storage tank).
6. Your pump will pump fluid until it runs “dry” – which will happen when the vapor space nitrogen enters the suction dip pipe. At this point you stop the pump and shut the discharge valve. You now must drain all residual fluid from eh suction side of the pump as well as any left in the discharge side of the pump. There will always be residual fluid left in the railcar since this is a top pumping operation.

That, essentially, is the basic manner I would prime and startup a pump installation such as yours. There may other details I have missed, but basically that is how I would propose the startup operation. I would still insist on having the basic NPSHa calculations made since these are essential to fixing the minimum nitrogen (or air) pressure that has to be imposed on the railcar. This would then be checked with ALL available railcars subject to this pump-out operation in order to ensure that NO RAILCAR MAWP IS EVER SURPASSED. Do not underestimate the safety or hazardous implications in this operation. Do not proceed to an installation without a serious and detailed Hazop involving plant personnel - especially operators.

I hope this helps out.

[kk911]

dear both,

do you have any idea/method to prevent leak/exposure while unloading the benzene rail tankers? it's top unloading.

thanks alot...