The other diaphragm pump
The double hose-diaphragm pump has inbuilt redundancy and sophisticated condition monitoring

Not all pump diaphragms are flat. The hose-diaphragm piston pump, with its tubular diaphragm, linear fluid path and isolation of the process fluid from the pump casing, was invented more than 30 years ago to overcome the shortcomings of traditional diaphragm and diaphragm piston pumps. Now the double hose-diaphragm pump improves even further on the original design.
Heinz M. Nägel
Like their predecessors, double hose-diaphragm pumps use a cylindrical flexible hose to transport the process fluid. Because pressure is applied to the whole outer surface of the hose element at the same time, and the hose remains open at all times, they count among the diaphragm pumps.

At the heart of the patented Feluwa Multisafe double hose-diaphragm pump are two hose-diaphragms arranged one inside the other. These create a double hermetic seal between the process fluid and the environment, though the pump needs only one of them to operate.

Unlike in a traditional diaphragm pump, the process fluid is not in contact with the material of construction of the pump head. Since the pump head normally makes up most of the weight of the complete pump, this isolation of the process fluid reduces corrosion, erosion and material costs when dealing with corrosive materials.

Mimicking the human body

A mechanically-driven piston pumps hydraulic fluid to the outside of the two cylindrical diaphragms. As the diaphragms contract with each stroke of the piston, fluid is squeezed out of the pumping chamber through a non-return valve. Then as the diaphragms expand in readiness for the next stroke, fresh fluid is drawn in through another non-return valve. The resulting flow path is linear and isolated from the material of the pump head, while the use of two diaphragms provides security against leaks. Peristaltic hose pumps rely for their pumping action on occlusion – squeezing the hose until the fluid path is completely blocked. This places a lot of stress on the hose. In contrast, the hose-diaphragms of the Multisafe pump are not occluded. There is simply a slight in-and-out movement with every stroke of the piston. This keeps hose stresses low, so the service life of hose-diaphragms is considerably greater than that of peristaltic hoses and traditional flat diaphragms.

The human heart and veins, with their elastic walls and non-return valves, form
a powerful low-shear pumping system that is long-lived and highly reliable. The
Multisafe hose-diaphragm design essentially mimics this biological system.

Monitoring for reliability

The hose-diaphragm pumps can operate without damage in the absence of fluid flow, making them intrinsically reliable. The use of both primary and
secondary hose-diaphragms also contributes to reliability. And for ultimate peace of mind, a range of instrumentation is available to monitor the condition of the pump. This includes checking for diaphragm leaks, monitoring the
suction pressure and the temperature of the
hydraulic fluid and gearbox, and a special system that detects leaking check valves.

The pump monitoring instruments can be integrated into the main plant control system, using either a point-to-point connection or a Profibus link, or even a telemetry system, to ensure that the information on the status of the pump is always available in the control room. Large Multisafe pumps also have their own programmable logic controller (PLC) system. This has a local display to show the pump status, and a touch pad for data input.

The primary hose-diaphragm (the one on the process fluid side) is hydraulically coupled to the secondary hose-diaphragm (on the hydraulic fluid side). The space between the two diaphragms is unpressurized. In the event that either of the two diaphragms leaks or fails, either process fluid or hydraulic fluid will get into the space between the diaphragms, and the local pressure will increase. This pressure increase can be detected by a sensor attached to the condition monitoring system, or alternatively leaks can be monitored through a visual inspection system.

Regardless of how a diaphragm leak is discovered, the pump can continue to operate without problems until a convenient opportunity for maintenance presents itself. This “early warning” system for diaphragm failure therefore combines safety with reliability.

Listening to the valves

Specially-designed detectors are available for monitoring wear in the pump’s check valves. The measuring principle is based on the analysis of structure-borne sound created by the check valves; the detectors can spot leaks between the valve seat and ball or cone while the drop in flowrate is still less than one percent. The sensor is fixed to the outside of the valve casing and is not in contact with the process fluid, so it can even be retrofitted easily to old pumps.

The device is known as the FVPMS (Feluwa Valve Performance Monitoring system). It provides precise information as to which of the check valves is leaking and might need replacement. FVPMS can significantly improve pump availability and reliability, with a corresponding reduction in maintenance costs.

Wide range of application

Multisafe pumps have a wide application spectrum. They are used to handle aggressive and abrasive fluids, including suspensions of different viscosities, but they are also specified for foodstuffs and aseptic applications.

The pumps are available with up to six pumpheads; combined with a wide range of pumphead sizes, this allows them to cover an enormous range of flowrates (0.1–600 m3/h) at pressures up to 320 bar and temperatures up to 200 °C. The maximum allowable suction pressure is equal to the discharge pressure.

The fact that the process fluid is isolated from the material of construction of the pumphead, even if one of the diaphragms should fail, means that materials can be selected to suit the flowrate and pressure only. The result is a pump design that combines high flowrates and pressure capabilities with gentle handling and great reliability. n