Pumps without electricity
A pump that offers virtually a
maintenance-free alternative
Think of pumps and pumping and the electric pump immediately comes to mind. However, there is an alternative; one that offers virtually maintenance-free operation, is easy to install and doesn’t use electricity.
Such a combination of benefits would seem to be the perfect solution for many industrial applications. Yet, although widely used in the USA, the mechanical pump is regarded in the UK and Europe as somewhat dated technology. This is surprising, because the mechanical pump is an inexpensive device, powered by steam, compressed air or other pressurised gas, that offers true fit-and-forget reliability. Although not suitable for all pumping applications, there are many areas where a mechanical pump could arguably be used more cost effectively than an electrically powered pump. The attraction of this type of pump lies in its sheer simplicity. There are no seals to maintain and its few, slow moving parts are not subject to excessive wear. As a mechanical pump is a selfcontained unit that uses steam or other pressurised gas for motive power, no electrical supply is needed, simplifying installation and making them suitable for use in wet or hazardous areas. A further advantage is that a mechanical pump costs far less to run than electrical pump sets. When started, centrifugal electric pumps cause mechanical stresses and peaks in electrical demand. A common solution to this is to opt for a continuously running pump with a modulating valve in the pump discharge, to throttle the flow. This minimises mechanical stress, but increases electricity consumption and running costs. A mechanical pump only operates when there is fluid to be pumped and, because its motive power can come from steam that is returned to the system, it has very low running costs.
How it works
The mechanical pressure powered pump operates on a positive displacement principle. Liquid enters the pump body through the inlet check valve. As the chamber fills, the float rises and operates a snap action mechanical linkage that opens the gas inlet valve and closes the gas exhaust valve. The linkage mechanism ensures a rapid changeover from filling to pumping.
As gas or steam pressure inside the pump increases above the liquid back pressure, the liquid is forced out through the outlet check valve. The liquid level inside the pump fails, the float reengages the valve changeover linkage which closes the gas inlet valve and opens the exhaust valve. Pressure inside the pump body fails, liquid enters through the inlet check valve and the cycle is repeated. Mechanical pressure powered pumps require only minimal amounts of steam or gas for power. Typically, only 3 kg of steam is needed to pump 1.000 litres of fluid. The amount of fluid pumped can also be metered by the addition of a cycle counter. Remote indication of pump operation is also possible by fitting a pressure switch linked to a relay to give a signal to a panel or process computer to warn if the pump fails to operate within a set time.
Applications
One of the newer applications for the mechanical pump is to remove condensate from heat exchangers, under all operating conditions. Poor removal of condensate from a heat exchanger, for example, can cause waterlogging and erratic temperature control. At design load steam pressure in a heat exchanger is usually sufficient to force condensate out. However, when process demand is low, the pressure in the exchanger may be less than the pressure needed to remove condensate. Condensate builds up in the exchanger and reduces its heat transfer effectiveness. The problem is usually best solved by installing a mechanical pump in series with the normal steam trap to ensure condensate is removed from the exchanger under all load conditions. The pump needs only enough suction head for condensate to drain into its chamber and start the pumping cycle. Although electrically powered centrifugal pumps are usually reliable and trouble free in many applications, condensate pumping is often the exception. Condensate from steam traps fails by gravity into a receiver tank and from here, pumped through the condensate recovery system. The temperature of the condensate in the receiver is usually around boiling point. As the condensate is drawn into a centrifugal pump’s inlet at a lower pressure, flash steam is produced in the pump, which severely reduces pumping capacity. Cavitation, caused by collapsing steam bubbles within the pump’s impeller, can also erode the pump and reduce its life. In many installations, the receiver and condensate pump are located below the steam system in a pit, which can easily flood, causing electrical pumps to fail. All these problems can be overcome by using a mechanical pressure powered pump, which is capable of handling condensate under all operating conditions. Mechanical pumps can operate even when completely submerged. However it is not just condensate pumping that can benefit from the mechanical pump. It is an ideal solution, for example, to problems that paper mills often experience with electric pumps used to recirculate lubricating oil on many large paper machines. Such machines have a forced lubrication system whereby an electric pump supplies oil to each cylinder bearing. The oil then drains to an electric sump pump situated below the paper machine, before being recirculated. The hot environment in which these sump pumps operate can cause premature failure of their gland seals and cause oil leaks. In addition to the high costs of maintenance and increased paper machine downtime, oily floors can be a major safety hazard. Such problems are completely eliminated by replacing the electric pumps with mechanical pumps, which have no gland seals and require no maintenance. Furthermore, these pumps are low cost and easily installed, needing only a compressed air supply for power. The mechanical pressure powered pump is a simple yet effective device that should deserve more attention from industrial engineers. When used in the right application there are few disadvantages and many benefits over electric centrifugal pumps. The mechanical pump is very much a product of the twentieth century, rather than a remnant of the nineteenth.
Gerald Bauwens, Spirax-Sarco
