Pumps in Series, Different Size

[ProcessHVAC]

We are commissioning a small train fueling facility and we are having some issues with a train fueling nozzle. Apparently, the manufacturer claims it is activated by vacuum and then pumps dead head pressure seals it, but we are finding it doesn't shutoff some times. The contractor claims the system needs a bit more pressure. The currently installed pumping system is 160 USGPM, 35 psi. Two pumps, duty and standby (Pump 1 and Pump 2). The system was started and works well, we tested the pumps and they operate as intended and we are getting the flow we wanted. I am thinking about using a series system to boost up the pressure. I have done pumps in series for HVAC applications but they are usually small pumps so I am trying to figure out actual operational details of putting these pumps in series.

Assuming the system needs a bit more pressure, my options are:

A) Increase the size of the impeller and boost up the pressure to around 47 psi, I am not sure if this is enough though.

B) Put Pump 1 and Pump 2 in series and boos up the pressure to around 70 psi, we have control valves for throttling so excessive pressure isn't a problem. Only problem is I lose standby pump.

C) There is another set of pumps (Pump 3 and Pump 4) they are the same model but with a smaller impeller (160 USGPM 22 psi) used to transfer fuel from tanker to below grade tanks. I am thinking about putting the two systems in series.

I would like to pursue alternative C, as it allows me to keep standby feature. The question I have is, I have heard that putting pumps of different size in series can be problematic, how can I overcome this?. Bypass with check valves around the second pump? Flow control and pressure is not critical as the system almost self-throttles itself at the nozzle and as I said we have globe and modulating ball valve in the system. I have a main plc doing pump start stop, etc and can reprogram it.

 

[DubMac]

Just don't know where to start with this one..... but I'll start with saying the first thing you look at with series pumping is whether or not the 2nd pump's suction side can deal with the discharge pressure of the first pump; seals, casing, etc. That's just for starters.

If you just want a general comment, then you've probably given enough information and can go on your way. If you are truly interested in solving your dilemma, the pros on this site will pound you for more specifics. Why don't don't you get off your butt and give them pump curves, info on the piping arrangement, motor sizes, SGs, etc.? A little effort on your end will yield spectacular results, I promise!

From a contractual standpoint, I wonder if the contractor (and others) aren't having you cover their design mishaps??

 

[Artisi]

Is it a pump problem or a fuelling nozzle problem? "we are finding it doesn't shutoff some times" - sounds more like an equipment problem rather than pump problem.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[itsmoked]

I tend to agree with Artisi, this may be a nozzle problem. Remember that locomotive fuel tanks can reach 10,000 gallons which equates to large surface areas, that lead to bursting seams, when they get over pressured a little bit. True, you have your pressure valves but they can fail and excessive available pressure can then be presented to the tanks - possibly ruining everyone's day.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ProcessHVAC]

Thanks for the responses, I will add upload a bit more information when I get back in the office. I am also a bit suspect of the settings of a control valve that might be throttling a bit too much.

 

[racookpe1978]

What is the "sensor" to the cut out valve?

That is - If the nozzle "sometimes" gets jammed all the way into the tank so that internal pressure rises (as more fuel is pumped in), would that unexpected excess pressure trip the valve?

Could it reset or overload the valve so that fuel doesn't doesn't trip off?

 

[MortenA]

About your pumps in series question, its actually the most common thing to different size pumps in series (e.g. a booster and a main pump) e.g. if the NPSHr of the main pump is high. Its when you try to different pumps in parallel it get tricky (and you might end up having on of the pumps doing noting at all).

Best regards, Morten

 

[LittleInch]

Agree there's something a bit odd about this nozzle but looking at your options,

A) Remember you would also need to look at and possibly increase the motor power, cable size, starters etc, not just replace the impellor

B) Doubling your pressure will have an impact on flow unless you have some sort of flow control device in which case you're wasting energy

C) Looks ok given the rated flow duty is identical. Yes just install a bypass with a check valve on the second pump so that one pump will work then start the second one

As others have said - look out for inlet side seal rating and make sure your lower pressure pump is the second pump so that it comes on second, but goes off first.

The issue about series is more for larger systems when either the inlet seal is over pressured or where you turn the second pump off but the first one dead heads for a while until the downstream system pressure decays - can lead to problems but not in your case by the look of it.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ProcessHVAC]

Thanks for all the responses. We have a bus fueling system in this same facility working at a lower operating pressure and it is performing well. In theory (according to the manufacturer) the nozzle is supposed to sense vacuum loss from diesel fuel tank vent and engage the shutoff system (has an internal venturi), the system seals shut using pumps dead head pressure against the valve. Only problem is sometimes it doesn't engage it some times.

I checked series alternative, and the smaller pump is only about 80% zero head runout flow of the bigger pump and orifice flow shouldn't be a big concern. Operating pressure of the pump is still acceptable. I still need to investigate the nozzle a bit more before recommending any changes. I would be disappointed if the system requires a higher operating pressure as the manufacturer/sales engineer never mentioned it during the design phase of the system and it was not published in their product's engineering data.

 

[ProcessHVAC]

So I was finally able to solve this problem, and would like the answer in case someone ever comes up with same issue. This was not a flow/pressure issue nor a nozzle problem, it turns out that the problem was that the owner modified the trains' diesel tanks to have a tee fitting off of the nozzle's tank adapter to connect to a fuel management system. So the autoshutoff system could not "sense" the diesel interrupting this vacuum and the autoshutoff was not activating. This was only done on one side of the tanks, which explains while the system was engaging some times, whenever we were fueling the train from the left hand side.

The contractor was right that by increasing the pump pressure and flow the system would work better, but not for the right reasons. When more flow is pushed through the vacuum is increased enough to offset the loss through the tee fitting. We will be talking to the owner about increasing pump/flow or fixing the tanks, cost of taking whole fleet of trains out of service might be way more than just increasing the pump size in short term. We will be doing a series system test to figure out how much additional flow we require to make it work. Nevertheless, the owner should fix the tanks to avoid having to oversize fueling systems.

 

[LittleInch]

Thanks for coming back to the forum, but I can't understand your system and your answer. Any chance you can sketch this out as I don't understand how a simple presumably no flow tee to a fuel management system affects your system.

where does this vacuum come into anything?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way