Tanker Truck Offload - Low Flow, Low Pressure Metering

[dbrowns]

Hi all;

Longtime lurker, now it's time for a question of my own.

Customer Objective: volumetrically measure fluid during a gravity offload/minor pump suction for custody transfer. They do not want power at this site.

Knowns:
Link

- Tanker Truck Size: 16' tall, 6,500 gallons.
- Fluid: jet fuel
- Truck offloads into a manifold, manifold is sloped at down to "receipt" pumps located about 200 ft away and 4' below truck bottom. Pump delivers fuel to tank with a height of 40-45 ft.
- Pumps are centrifugal and rated for 200 GPM
- New 300 GPM Rotoprime pumps are slated to be installed next year per conversations with personnel
- Truck bottom to centerline of manifold is about 8 inches (obviously little head)
- Average offload rate with pumps running is 100 GPM
- Pipe is 4"
- Operators brilliantly vent the air (from hoses, vortices, uneven compartment drop down rate) with an air release valve to a spit can to prevent the pump from air locking.

Design Considerations:
- Check the NPSHr of existing pumps/new pumps. I don't want to starve them and create a cavitation situation.
- Air elimination will be needed. I've thrown out the air eliminator-meter-air block valve combination Liquid Controls sells as there isn't enough head to open that valve. I'm now looking at 50 gallon tanks with a vent line extending above the height of the truck. The customer does not want power at this site, so level switches are out of the picture.
- Verify that there is enough head to deliver fuel through the air elimination tank and meter.
Link


Questions:
- Would the vent line be better suited with a pressure/vacuum vent? Or piped to a spit can with a check valve in the line to prevent air from being pulled in?
-Will this even work?

I'm relatively new to the post-academic engineering world. I've designed a few systems with a meter/air eliminator downstream of a pump or a skid with the large, level-controlled air eliminator upstream of the pump...but I haven't come across a system like this where the only means to move fluid through a meter is via gravity and on the suction side of a pump. Any help or guidance you can provide is appreciated.

 

[georgeverghese]

Never seen this configuration with meter on pump suction with so little head available - have doubts if you will ever get the NPSH required. What are these rotoprime pumps - do they have some kind of inbuilt suction head booster pump ?

 

[chicopee]

Any chance that tanker trucks can be temporarily retrofitted with pumps to offload until the new pumps are installed? When the new pumps are installed, you may want to build ramps in order to increase head and to satisfy the NPSHR of the pumps.

 

[dbrowns]

georgeverghese: Yeah, it's an odd configuration. The client has a limited budget and is convinced that there is a low cost solution. Rotoprime pumps are self-priming centrifugal pumps manufactured by Gorman Rupp (Link
). From my experience, these pumps work well with situations in which air is present in the upstream piping. The pump can be ordered with an integral air eliminator that vents air to a recovery tank. It would be nice to put the meter downstream of this, but the client does not want that.

chicopee: That's a good question. I'll ask the client. The ramps are a good idea, but I feel that there will still be an issue with the location of the air eliminator and meter relative to the pump. If the suction is too high, air could be sucked in through the vent line. The vacuum rating on the vent would have to be high enough to prevent air from being introduced and low enough to prevent collapse of the air eliminator. I'll have to check BPVC and manufacturer's literature for vacuum ratings.

 

[chicopee]

With tanker trucks temporarily retrofitted with transfer pumps air or fuel vapor should not be an issue unless the truck operators get careless and empty the tanker trucks dry.
Before the final installation discuss your concern with the manufacturer of the air eliminator. If suction pressure is too low increase the pipe size. You still have to theoretically calculate the line pressure to determine its magnitude at the air eliminator for any potential problem.

 

[LittleInch]

Ok,

A couple of points spring to mind.

1) I don't understand why your vent arrangement comes back down to ground level with a U bend. In theory you could turn this into a syphon or end up with liquid in the bend which then gets forced out when you connect the next truck onto it.

One thing to maybe do is just have the vent point straight up, but introduce either a wider section at the top ot an orifice plate as otherwise when the vent fills up it can get a significant velocity and end up geysering out the top. This happened to a friend of mine who was doing the same thing with a buried tank which was being filled by rail cars - same principle.

2) your flows seem to be all over the place - you quote current pump is rated at 200 gpm, new one 300, but average off load is 100? That's 65 minutes to offload a truck load which seems very slow. My normal reckoning is 25-30 minutes to offload an entire vehicle.

3) You don't say much about the meter. At that very low pressure / differential pressure, you probably need something like a mag flow or unltrasonic - I know mag flows can work off batteries, but if you're using a mechanical meter you need to be on the discharge side of the valve otherwise you'll probably end up with vapour when you're at the low level in the tank truck.

Your existing set up link didn't come out so if you paost that again it would be good.

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