Keeping two parallel recip pumps out of synch phase

[Scipio]

Wondering if anyone's got an idea how to keep two hydraulic diaphragm style pumps, running in parallel, out of synchronous phase. Basically I've got one existing unit in service (drawing sour condensate off an inlet separator and injecting it downstream of a gathering compressor), but we've brought some new wells in and it's just barely holding it's own in capacity terms right now. Operation is fairly simple, pump runs continuously at 100% stroke on a fixed speed motor. A level controller on the separator operates a recycle valve, so the pump just recirculates condensate back to the separator inlet unless it's seeing a significant level in the separator, at which time the recycle valve closes and the condensate is directed to sales. The condensate is still relatively 'live' in that it still contains dissolved high vapour-pressure gas, we run continously to prevent vapour locking.

Now, I'm planning on installing a second pump to operate in parallel with the existing. All hydraulics are good, with one possible exception. My only area of concern is the acceleration head. With two independant pumps operating in parallel, I'm worried about their suction/discharge strokes being coincidentally synchronized and driving up acceleration head (as opposed to being 180° out of phase and reducing it). Under normal circumstances I'd drive the two pumps off a single motor through a reduction gear with two output shafts, so the cranks could be hooked up out of phase, but for various operational & maintenance reasons I want to keep these pumps running with independant drives.

So, the question becomes, anyone have an opinion as to whether or not my concerns are justified? If they are, is there a way to ensure the two pumps don't operate in synch? All precautions to minimize acceleration head have already been built into the suction piping, short of installing a suction stabilizer - I've made provisions for one if required but I prefer to design around them.

 

[Kawartha]

scipio,

Interesting problem. I am not aware of any mechanical or electrical method to ensure that two separate machines (driven by induction motors - I assume) will operate continously in or out of phase with each other. This may be possible with Synchronous Motors. Perhaps you should post this question on the electrical thread also. It seems to me that if there is a solution, it will be electrical. If you are successful in finding an answer, please post it in this thread.

That leaves two options, which you already pointed out:
1) Redesign the system, and add necessary dampers on suction side of the pump. You may not require additional dampers on the discharge.
2) Drive both pumps off one driver. You could use quick disconnect clutches to take either pump off-line, if that is your concern.

 

[ccfowler]

If you are driving two "identical" pumps with induction motors, it is likely that they will operate at slightly different speeds. This will cause them to operate at some reasonably constant changing between in-phase and out-of-phase operation. Getting two synchronous motors to operate at some particular phase relationship could prove to be a fairly complex and expensive issue.

I would not actively pursue any of other options until I first resolved the costs and practicality of using pulsation dampers at the suction and discharge of both pumps. After this, I would look into powering both with a single motor. Then I would consider the other more "elegant" options.

I doubt that anything beyond pulsation dampers at the suction and discharge of both pumps will make economic and practical sense, but I would very much like to learn how your findings develop. Please keep us informed!

 

[Scipio]

Kawartha,

Assumption correct, motors are both 1800 rpm, 3/4hp induction motors. Interesting idea with the clutches, however that option fell by the wayside on Friday afternoon when operations asked me to set up the two pumps so #2 comes on automatically if #1 can't keep up. First thought was to run the #2 pump continually, and use a electro/pneumatic stroke positioner to change the stroke from 0% when not needed to 100% when needed, but Pulsafeeder says the upper suction pressure is too high for their stroke positioner to work properly.

ccfowler,

Good point on the slip with inductions, I knew they'd be running < 1800 rpm, forgot that they'd likely have different speeds though so the pumps will probably be slipping in and out of phase, and I'd have, at worst, a transient problem with acceleration head.

Think I'll roll the dice and just leave provisions for suction stabilizers on the piping if it proves to be a problem. If I post it in an electrical thread I'll probably just wind up looking at a switchgear price tag that would pay for five suction stabilizers anyway. Thanks for the input, good to know it's a problem worth a second thought.

 

[d23]

Scipio

Have you considered a magnetic coupling for one or both of the pumps? A vibration switch could be used to allow a little slip. The price should not be very high for a 3/4 HP unit.

Good Luck!