by Andreas Kneer

Plastic pneumatic diaphragm pumps are used widely in chemical, semiconductor and pharmaceutical applications, as well as other industries that require sanitary operating atmospheres. These pumps are ideal for these applications because of their almost universal chemical resistance-due to housing materials constructed of PTFE (polytetrafluoroethylene) and conductive PTFE-and high wear resistance-thanks to housing materials that use PE (polyethylene) and conductive PE.

With the increasing degree of automation found in plant construction and operation, functional reliability is playing an enhanced role with respect to a plant's total cost. Therefore, it is crucial that the right components contribute to a plant's operation (for example, comparatively low-cost pumps that are small, but effective) as unscheduled plant shutdowns and production downtime quickly result in high costs.

The Challenge

Finding the proper pump for a specific production process requires a lot of legwork. Different pump modes of operation must be considered-from positive displacement to centrifugal to peristaltic-along with workload, flow rates, working pressures, etc.

One style of pump for chemical, pharmaceutical or semiconductor operations is pneumatic diaphragm pumps. These pumps are commonly used in static-fluid energy machines characterized by energy conversion in an enclosed working space with periodic energy transfer. Appropriate to their mode of operation, these pumps are classified as lift-displacement machines. Features of pneumatic diaphragm pumps are a gentle product displacement, dry-running and overload-proof operation, self-priming, insensitivity to solids, infinite variability and easy-to-operate functions.

One key to pump efficiency is its ability to provide a uniform delivery flow. To achieve this uniform flow rate, along with design-related, low-residual pulsation, pneumatic diaphragm pumps are usually designed as externally circulated double-diaphragm pumps (Figure 1). Two identical diaphragms are arranged opposite of each other, with each separating the air chamber from the product chamber, while they are interconnected by means of a piston rod.

Figure 1

The pressurized "drive air" created by the pump's operation flows into the chamber behind a diaphragm. To enable the pump to perform its working stroke, the introduced compressed air must overcome the pump's internal friction (static/sliding friction on the sealing and bearing elements) and the diverting forces of the diaphragms, as well as the product-sided back pressure. The continuous flow of drive air causes the pressure behind the membrane to increase until the counteracting resistance forces are overcome. The diaphragm then displaces the medium from the decreasing product chamber and performs its working stroke. This forces the transported medium to be pushed out of the pressure side. To ensure adequate operation, the diaphragm is pressure balanced during the delivery stroke.

Meanwhile, the opposite diaphragm, which is connected via the piston rod, moves toward the stroke-performing diaphragm. The expanded air behind this diaphragm is allowed to escape. The medium then enters the enlarging product chamber on the suction side, allowing the diaphragm to perform its suction stroke.

The crux of this pneumatic double-diaphragm design dictates that maximum delivery volume and the maximum possible delivery head (i.e., back pressure) are the main parameters of the pump's operation. The dimensioning of the necessary wall thickness, product passage cross-sections and valve lift grow out of these design parameters. Therefore, the delivery volume of pneumatic diaphragm pumps can be influenced via the form and dimensions of the displacement chamber, the volume-changing kinematics (or diaphragm stroke) and the number of working strokes per unit of time.

Additionally, the design of the product chamber and determination of the diaphragm stroke are mainly aimed at achieving a long diaphragm life. Ideal dimensions are large diaphragm diameters combined with small diaphragm strokes. The geometrically optimized design of the air or product chamber for the minimization of dead volume in the respective final diaphragm position positively influences the pump's efficiency.

The number of maximum possible working strokes (in addition to drive and back pressure) depends on the air passage volume to be filled behind the diaphragms. The dimensioning of the air passages must be optimized. On one hand, this volume should be minimized to avoid unnecessary dead volume. On the other hand, the air passage cross-sections within the pump and the air-control system must be dimensioned for the expanded, pressure-less air, as this takes up a larger volume than the pressurized drive air. The expanded air behind the diaphragm performing the suction stroke must be able to escape quickly enough to prevent the entire system from slowing down by an inefficient "air cushion."

Because of the pump's designed-in internal flow reversal, pneumatic diaphragm styles require a control that provides the diaphragms with an alternating supply of compressed air. In modern combustion engines, injection systems distribute the fuel/air mixture to the individual cylinders. Air-control systems in pneumatic diaphragm pumps alternately introduce pressurized air into the air chambers behind the diaphragms, or discharge the expanded air to the silencer.

The two types of air-control systems are direct and indirect. Direct air-control systems are characterized by how the distance of the control travel by the pilot piston corresponds to the diaphragm stroke. Direct air-control systems are normally used in small sizes (piston rod=pilot piston) and are arranged centrally between the diaphragms.