Minimum dry vacuum or minimum suction lift of a vane pump

[boom123]

Hi,

I have some questions with regards to a diesel pumping application. I am using a vane pump to suck fuel out of a main storage horizontal tank. The layout is such that the the pump is installed about 2 feet above the tank bottom and is connected via. a 2inch SCH 40 pipeline and antispihon valve, other fittings to a suction tube installed on top of the tank with the help of extractor coupling. Because of this arrangement the fuel level varies with the reduction in quantity of fuel remaining in the tank. The tank height is 10 feet and the Antisiphon valve is installed 1 feet above top of this tank. The Antisiphon valve has a setting of 5-12 feet opening head and is currently set at a crack opening head of 5 feet (I suspect this is for pure water with SG = 1).

The pump was selected to suck the fuel out of the tank using this arrangement and the manufacturer has specified a mimimum dry vacuum at pump suction to be about 18 feet of diesel. Now while commissioning the system, the pump is not able to suck fuel out of the tank below 4 feet liquid level ( i.e. only the top 6 feet of fuel is sucked out, remember the tank height/diameter is 10 feet).

I did an analysis and calculated the head loss in friction on the suction side of the pump ,at the set flow rate of 35GPM, to be about 4.5 feet. In addition the Anti Siphon Valve requires 5 feet of head to open. In all the Friction Head (incl. siphon valve crack opening head) is about 9.5 feet in the system.

I fundamentally calculate the static suction head/lift in any pumping system as the difference between the liquid level and the pump suction. In this specific situation the Static Suction Head varies as the liquid level is about +8 feet when the tank is full and the liquid is above the pump suction. The the static suction lift is -2 feet when the tank is just about to be drained. Is this the correct determination of static suction lift even if the highest point in the entire system is about 9 feet above the pump suction. Does the pipeline routing matters in the suction lift calculation or is it always the vertical difference b/w the liquid level and pump suction irrespective of the routing of pipeline b/w the suction pipe inlet and the pump suction?

I want to check if I am using the right suction lift (-2 feet)for this application? I am considering that the pump will always be primed and the system be hydraulically balanced i.e. the Antisiphon valve will hold diesel in the suction tube installed inside the tank.

 

[chief]

Fit a 2 ft loop on the suction side of your pump and your pump will always be flooded and provide you with a suction capability on start up.
A better installation would have been to fit a foot valve in the tank bottom pipe suction, but this is not now possible with a tank full of diesel.

Offshore Engineering&Design

 

[boom123]

Thanks chief for the valuable solution.
However, I wanted to check if I am correctly determining the suction lift for this application.

Is the suction lift the maximum lift required in the hydraulic system ?
Or
Is it the vertical difference in the liquid level and the pump suction irrespective of how the in between piping is routed?

 

[BigInch]

Would you mind posting a diagram of that system you're trying to describe.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[boom123]

BigInch,

pl. find attached the diagram of the system. I have included some details. The numbers are actual as applicable to the problem.

Thanks!

 

[BigInch]

So this antisiphon valve is acting (I believe) like a spring loaded check valve. I'm not really sure what it is doing in the system, since you have another check valve just before the pump. I think it might have been better to just put one check valve at A, like a foot valve at the bottom of a water well pump.

To start flow, yes the pump will have to draw through the entry head loss entering the pipe, the 11 ft A-C going up + 5.5 ft siphon valve head loss + head loss through the 2" + head loss at the second check valve and the head loss of the ells. Once the flow goes over the goose neck, you could get some help as the downcomer fills and adds the 10 ft of head back into the pipe run, to assist with suction pressure, but not 'till then.

I suspect that the diesel in the vertical run A-C is falling back into the tank whenever the pump is stopped, forcing the siphon to be re-established whenever it is started again. When the tank level gets too far down, it might be too much head to be able to restart.

BTW isn't that a maximum, rather than minimum dry head, and there is there a vent somewhere in that tank, right?

First thing to do is lower the opening force as much as possible on that antisiphon valve.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[boom123]

BigInch!
Thanks for your critique and valuable analysis.
The antisiphon valve at the top of tank is installed to prevent the siphoning of diesel should the 2" pipe b/w this valve and the inlet of pump breaks. The other check valve is installed to prevent any air from leaking inside the pump from the suction side.
The Antisiphon valve is supposed to work like a foot valve as it should be able to hold the liquid in the vertical suction pipe if there is no leakage in the system.
Your analysis of the initial priming upto 11 feet A-C and additional losses is quite understandable. Essentially the pressure generated from the drop from point C to the pump suction to point B will definately make up for the same head required to lift the liquid from liquid level to point C. I also kind of suspect that the diesel is falling back from the vertical run A-C. Currently the antisiphon valve is set to open at minimum setting of 5.5' required to overcome the vacuum/head generated by free fall of diesel from C-B should the pipe break at pump suction.

The minimum head of 14" of Hg or 15' of water is what is specified by the pump manufacturer. And it is called out as minimum only!! I also dont understand this specific parameter. But the pump supplier tech. support told me that this is the pump capability to suck fuel out....I will have to dig deeper into exact meaning of this parameter..

The tank is vented to atmospheric right now. I was thinking if lowering the pump will help in reducing the suction lift by adding more pressure from vertical drop from C-B. Any suggestions on this?

 

[Zapster]

Who is the manufacturer of the anti-siphon valve? Per the manufacturer, is this the preferred placement for the valve when it is installed in pump suction line? Is the diaphragm material in the anti-siphon valve compatible for the product in the tank? What do you mean by ”The other check valve is installed to prevent any air from leaking inside the pump from the suction side.” What type of check valve is this and what is cracking pressure of the valve?

Nowadays doesn’t this type of installation typically use a submersible pump unless a single pump is used to services multiple tanks? Have you considered a solenoid controlled vacuum break instead of an anti-siphon valve on the top of the tank?

 

[BigInch]

I've never seen an antisiphon valve installed on a diesel tank... but I've not seen everything yet. They are more common on irrigation and water supply systems to prevent flow back from the irrigation from returning to the drinking water supply. It seems like an anitsiphon valve is an excessive safety feature for this installation. Is there such a danger for breaking that 2" pipe? This pipe has no pressure, or high temperature. Is there some other extreme danger of breaking the 2" pipe? If so, I would have made the 2" XXS. I would think a leak in the tank would have an equal or greater probability, so what safety provisions have been installed for that possibility?

What is conneted to the discharge side of the pump?

Do you have an internet link to see the technical information for this pump?

It sounds like the supplier is simply saying that the pump can develop at least enough suction to lift water 15'. It might be able to do more, but they won't guarantee that. If they are "stretching" the truth here, that indeed could be the problem. 11' + 5.5 is higher than that.

When you hve high tank levels, does the pump work well until the tank level gets to 4'?

When you have low tank levels, does the pump start the flow, pump out the diesel in the 2" pipe and then lose flow immediately after that?

Lowering the pump will always help available suction head, but that won't do anything for the 11' initial lift. The diesel must still make it over the gooseneck. If you can easily lower the pump, you could try it. If the problem is simply head loss in the pipe length, it could help.










**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[boom123]

Hi BigInch,

Pl. refer bleow for the responses.

1) The Anti-siphon valve is used a postive shut off should the fuel line accidently break and to avoid the tank from being siphoned out. This is required as part of the Fuel handling Code to provide a positive shutoff on fuel storage tanks.

2) At the discharge of the pump a 1" automatic fueling nozzle with 12' flexible hose is installed.

3) I have attached the pump spec. sheet the pump model in question is FR311.

4)Minimum dry vaccum of 14" of Hg is stated in the attached spec. sheet.

5) The pump works well at high levels. It drops and kind of stalls when the fuel level in tank is about 4'.

6) I havent infact seen the problem myself. but I have been told that the pump struggles to get the fuel out below 4'.

It seems right now the pump is working at its max. suction capability once the liquid hits 4' level in the tank. May be you can throw more light on this issue..

thanks a ton!!

 

[BigInch]

Looking around I did see one fuel transfer tank diagram with an antisiphon valve, so I guess they're ok there.

Dry vacuum seems to be what it can draw without using any liquid for sealing.

Help??? I'm not sure how much I'm helping. I'm running out of ideas on this! Theoretically, if you believe the pump data, it looks like it should be working, but running until it reaches 4' level is very suspicious and would indicate a suction head limit, or maybe a discharge head limit has been reached.

Time to look for problems in the suction line. Antisiphon valve set too high, or not working properly, something blocking the suction line?

Now for discharge. Can you measure the discharge pressure? Or alternativly, what is the height of diesel in the vessel you are filling when this pump stops?



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

Still thinking about this.

What altitude is this pump set at?
Its possible that altitude could be affecting suction lift.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[chicopee]

Looking at your diagram,why didn't you have the tank outlet pipe installed at the bottom of the tank? I have never seen a diesel fuel bulk storage tank the way you have it. Also, I did not see any opening in your diagram that would prevent partial pressure in the tank as the liquid is drawn out. It is possible with ahe slight negative pressure and friction loss in the pipe at whatever GPM the pump may have, it is having a tough time to draw out diesel fuel.
Also, get the pump performance curve to see what head is the pump at with 35 gpm. Cavitation on the pump intake should not be an issue with diesel fuel oil.

 

[BigInch]

Presense of vent is stated 6 Apr 09 21:33

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/