News Archive
Issue 8 August / September 2008
Whole life cost? Look at the detail of your pump
As soon as the universal joint in your progressing cavity pump
starts to wear the effect will be immediate:
- Reduced operating efficiencies
- Seal leakage
- Excessive rotor movement causing high stator wear, this will
reduce service life dramatically
Therefore, it is worth looking in some detail at the universal
joint design of the pump you are about to purchase.
The majority of progressing cavity pumps specified today still
utilise the original ball and pin universal joint design to compensate
for the eccentric motion of the rotor. This simple design has stood
the test of time, particularly for light duty applications. It is
functional, low cost to manufacture and quick and easy to repair.
Variations of the ball and pin joint do exist. The Robbins & Myers Moyno pump utilises a connecting rod and ball, precision machined
and hardened for best fit and joint performance.
You will also see bushed designs, primarily to save replacing a
worn joint and rod. However, in practice, bushing degrades the contact
points because:-
- Tolerances accumulate
- The hole in the connecting rod bush is hourglass shaped, rather
than the preferred angularly elongated slot. This is to eliminate
the need to index the bushing to the rod axis
- The load-bearing outer surface of the ball is no longer a hardened
surface
Bushing also increases the cost of the universal joint assembly
and maintenance time.
The Moyno pump rod is supplied as a precision and hardened component,
for longer life. Subtly, but crucially, it is also pre-drilled at
90 deg, offering users repeated ball and pin connection. Maintenance
of the Moyno component is literally at zero cost and down time is
reduced.
Another design you will see is the flexible shaft, whose primary
purpose is to eliminate moving parts from the universal joint and
reduce cost. Sounds good?
It is, however, a light duty solution, the thickness of the shaft
being limited due to its requirement for flexibility. Hence it offers
limited torque capability and therefore pumping loads. The shaft
strength, in practice, is also weakened by surface damage due to
foreign bodies in the fluid and pitting or corrosion. Additionally,
the flexible shaft requires the length of the pump’s footprint
and servicing space to be dramatically increased.
If your application is light, the fluid consistently “clean”,
and pumping pressures stable, then all designs will perform satisfactorily
for you. However, factors such as variations in solids content,
viscosity, pressure, possible corrosion, all put extra loads and
torque requirements on your pump’s drive mechanism.
It is clearly vital to understand your application to determine
how robust you need your ball and pin joint to be for the lowest
whole life operating cost of your plant!
To go one stage further, if your application is arduous: for example,
pumping 30% solids sewage cake sludge, or requiring pressures 20,
30 even 100 bar, then a gear joint universal coupling design should
be considered.
Please call for further details of this heavy duty design Tel +
(0)23 80760000
If it’s not working…
Your supplier’s initial pump selection should eliminate problems.
In practice, repositioning pumps and changes to original duty conditions
can cause the designed service life / pump performance to change.
Here is a general troubleshooting guide to aid corrective action
and save you time.
Pump does not rotate
Check motor / inverter, selection and installation. Check for any
foreign matter in pump. Check for excessive static friction in new
rotor and stator. Check for stator swell due to chemical attack
or high temperature. Check for solidification of settled liquid
within the pump. Check gland packing is not too tight.
Pump does not discharge
Check suction/ flooding/ excessive suction lift. Check for air leakage
into suction pipe. Check pump is primed. Check stator or rotor for
excessive wear. Check direction of rotation.
Discharge output low or fluctuating
Check motor / inverter, selection and installation. Check for foreign
matter in pump. Check for excessive suction lift. Check air leakage
into suction pipe. Check for stator or rotor for excessive wear.
Check seal or packing leakage. Check pump speed is not too high.
Check liquid viscosity or SG is not too high.
Excessive stator or rotor wear
Check for stator swell due to chemical attack or high temperature.
Check settled liquid has not solidified within the pump. Check suction
lift. Check dry running. Check discharge pressure. Check pump speed.
If you should have site specific questions please do not hesitate
to ask. We are here to help. Tel + (0)23 80760000 or email rmsales@robn.com
Best regards
David Johnson
Business Development Manager