What are piston and what do they do?

A piston is the part of a pump that actually pushes the solvent through the system. An HPLC system generates considerable backpressure, so a piston meets very high resistance in its day to day life. It has to have extremely high mechanical strength down its axis, and be chemically inert. They are made of industrial sapphire, which gives them this strength and resistance to chemical attack, but as a consequence they are not cheap!

At one end the sapphire piston is mounted in a stainless steel rod. In operation, the rod is pushed forward by a cam on the pump motor shaft, forcing the piston into the pump chamber and displacing solvent into the HPLC system. A one-way flow through the pump chamber is ensured by inlet and outlet check valves, and when the pump reaches the end of its stroke, it is forced back by a strong spring which keeps the steel rod in contact with the cam on the motor shaft.

How to tell if a piston is broken.

When a piston breaks it usually snaps part way down its length. The end which has broken off is pushed forward by the other half, but does not return when the spring forces the piston back at the end of its stroke. Hence at the "wet" end the piston does not appear to be moving and no solvent is displaced.

For a twin piston in parallel pumphead (i.e. both pistons operate independently and out of phase, with their outputs combined) the flow rate is reduced to 50% if a piston is broken.

For a single piston pump, there will be no flow at all.

For a twin piston in series pump (i.e. Solvent flows through both pumpheads, with check valves only on the primary head) almost no flow at all if the primary piston breaks, and only slightly reduced flow but major pressure cycling if the secondary piston breaks.

How to replace a broken piston.

Before you do anything, please bear in mind that these are general guidelines only, and not definitive service instructions for all makes of pump. It is not hard to change a piston but you must be careful. A piston is extremely strong down its axis, but it will snap like a pencil if you bend it, even a little!

To get at the piston you need to remove the pumphead. These are usually made of solid stainless steel, and can be deceptively heavy. If there is a window in the pumphead, run the motor until the piston is as far back in the head as possible before you dismantle it. It reduces the force of the spring, and also the risk of breaking the piston if it is not already broken.

Loosen the bolts evenly, so that the head moves forward without twisting sideways, and supporting its weight as you do so. Once it is free, draw it forward and clear of the piston. You will see the piston or its remains sticking out of the pump body. If the end is sharp and jagged it probably is broken! If it is smooth and rounded it probably isn't!

To remove the piston from the pump, grasp it firmly with your fingers and draw it out slowly. It will normally be mounted in a steel holder, with a roller on the far end to ensure smooth travel along the cam. Pull out the whole thing, but bear in mind that the back end can be quite heavy. The piston unit will be retained loosely, possibly by a circlip, and can then be removed, inspected and replaced if necessary. If a piston is even slightly scored or scratched it will wreck a piston seal in no time, so if in doubt, it is best to replace the piston seal.

When replacing a broken piston be especially diligent to flush out all the broken fragments from inside the pumphead. Reassembly is relatively straightforward once you have dismantled the head because you will know where everything goes.

However, please remember three points

• Always fit a new seal when replacing a broken piston. The old one will almost certainly be shot to pieces.
• Be prepared to push quite hard to get the piston through the seal when refitting the pumphead, but bereally careful to push straight!
• Make certain the pumphead bolts are tight before starting the pump.

How not to destroy a piston prematurely.

Unlike other parts of the liquid end of a pump there is not much that you could do to break it while it is still in the pump. So if it goes it is almost certainly not your fault. However you must be really careful with the spare! If it comes in a rigid plastic tube, keep it in there, and when you do need to get it out, keep it somewhere protected from impact or rolling off the bench until you need it. Most pistons are broken outside of the pump, or during dismantling or reassembly.

Don't forget. If you have problems during the repair or wish to discuss it first, please call us on 01634-294001.or email : stuart@laserchrom.co.uk