OPERATING A ROTARY LOBE PUMP AT SHUT OFF HEAD

[Berko]

Dear All,

I had a recent discussion with an end user regarding operating a Rotary Lobe pump at Shut Off Head.

The pump is a 4" model & is pumping 6-9% Primary Sewage Sludge (with the possibility of fine sand entrained that gets through the de-gritting process).

Power at shut off head is approx 32 kW.

The customer is thinking of incorporating into his operation regime, running this pump at Shut Off head for between 6 & 10 minutes per day.

I believe this is not a good operating condition to implement.

Not only is there likely to be excessive noise & vibration created (which will reduce seal, bearing & rotor life), but also large amounts of re-circulation within the pump casing. Here if sand is present, it will promote premature wear of the rotors & casing.

Also with no where to go, the small amount of liquid in the casing is subject to 32 kW of energy, which will quickly turn into heat energy. I suspect after 6-10 minutes of operation in this condition the sludge may boil (bad news).

I would like to throw this one open to the worlds pump engineers for advice/comment/similar experiences.

Berko

 

[ccfowler]

If the 6 to 10 minutes are the cumulative time of numerous momentary episodes, then the damage will probably not be instantaneously catastrophic, but pump life will be needlessly shortened.

If the 6 to 10 minutes is to be the actual time of each episode, then it would be best to plan on having multiple stand-by pumps installed (with generous access provisions) to permit continued system operation while mechanics get to hone their skills for rapid replacement of failed pumps.

It would be helpful for the mechanics to study NASCAR pit-stop techniques since the failures will be very rapid. More than one pump may be expected to fail during each of these "entertaining" 6 to 10 minute intervals.

Shielding, drainage, and ventilation provisions should be particulatly generous and well considered both for their performance and for rapid access for pump replacement activities.

Keeping a warehouse full of spare pumps (new or rebuilt) should also be part of the operating plan.

Getting beyond the above light-hearted comments, I must ask: Why anyone would deliberately operate a fixed displacement pump without a discharge side relief valve?

If the client doesn't believe in relief valves for positive displacement pumps, are the motors to be similarly installed without protective relays or circuit breakers? If so, then replacement of damaged (burned out) motors should be similarly anticipated.

 

[Berko]

Thanks ccfowler,

My thoughts exactly.

Even if the pump was fitted with a "return to suction relief valve", surely if the fine sand in the sludge doesn't wear the pumps rotors & casing out with all that re-circulation, then the relief valve seat will wear & become ineffective.

If this happens, the leaking seal face will promote re-circulation back to suction during normal operation which at the very least will increase power costs & increase operating times.

Berko