TechArchive

Original article date: May 2000

Where water levels have to be controlled, an innovative solenoid-operated coaxial valve has allowed automatic self-priming water pumps to work reliably and almost independently

In the past, water pumps have needed to be manually primed at regular intervals. Now, a new smart priming system from Sterling Pumps is a fully automatic vacuum priming system which will operate from dry condition to a suction lift of almost 10m at sea level. The system is then able to maintain intermittent flows without the need to manually re-prime the pumpset. The unit can run dry without any damage, useful where continuous monitoring is just not possible.

The reliability of the pumps has been increased considerably by replacing a conventional lateral solenoid-operated valve with a coaxial solenoid valve. The valves can handle water with high levels of solid contaminants like silt, and are used to ensure no water reaches the vacuum pump, which forms part of the main pump’s priming system.

There are difficulties with conventional valves for this application. Small pilot orifices are needed to assist the diaphragm or piston open and close. They may only be a millimetre in diameter but they easily get blocked. The solids in the water often obstruct the seal between the valve piston and its seating. If water is allowed to reach and damage the vacuum pump, it needs to be replaced, at around (UK pounds)1000 a time.

The direct acting coaxial valves from KV, on the other hand, have no pilot orifices. They operate instead by moving a cylinder through which the working medium flows. This arrangement makes the valve self-cleaning and ensures dependable operation, even in the presence of heavy contamination.

Also, because the coaxial valve spool is balanced, the power needed to actuate the solenoid valve is very small, even at high media pressures. The balanced construction therefore simplifies the design of the control system. Also, unlike conventional solenoid valves, they can withstand up to 16 bar back pressure.

A great advantage for this application is that the valve has snap-action operation, which is important for the self-priming water pump. A ball valve could also be used if all that was necessary was to guard against contamination, because it also has no internal orifices. But electrically operated ball valves take 50-60 seconds to work, which could mean small amounts of water carrying over into the vacuum system. Again, this probably would mean that the vacuum pump, which should only ever see air, would have to be replaced. KV’s coaxial solenoid valve snaps into action after 100ms, thus ensuring the reliability of the pump.

A conventional solenoid-operated valve from KV’s high reliability range further protects the vacuum system of the water pump. The valve is controlled from a level probe, and operates automatically if the water level in the pump’s break tank reaches an excessively high level. Again, this guards against water entering the vacuum system. A proximity switch can be installed in the valves to send a signal to the controller to give positive confirmation of the valve position.

The self-priming water pumps are more reliable because of the coaxial and process valves and less maintenance is needed. Originally the coaxial valves were developed for a machine-tool coolant system and are used to working media, which have solids. This greatly increases the pump’s ability to deal with contaminated media in the construction and waste water industries.

These new coaxial valves come in 2/2 and 3/2-way solenoid and external pilot versions, and KV can supply them for use with media at pressures up to 500 bar. The types range from small valves for use in dosing applications, to robust flanged high-flow types used in the petrochemical industry. There are also valves certified for use in hazardous areas.

• Sterling Pumps
• KV

May 2000