How to select a diaphragm pump?

Why select a pneumatic diaphragm pump?

1. Diaphragm pumps are selected when the user needs a pump that is capable of running dry, or in other words, when there is a possibility that the pump may not be monitored and the fluid being pumped could be depleted. The diaphragm pump will simply go into a snore mode. Any rotating type pump, a centrifugal for example, if left without fluid passing through the unit, would result in a damaged mechanical seal with potentially costly downtime, spares, and danger to personnel and environment.
2. Air Operated Diaphragm Pumps are also selected when there is a need to transfer Toxic, Flammable, Acidic or Alkaline Chemicals or Slurries. The sealless diaphragm pump design enables one to safely and simply handle difficult fluids.
3. Pneumatic Diaphragm Pumps are ideal for selection when there is a demand for simple, safe, low-cost plant automation. Close a valve ont he discharge pipe and the pump will simply stop, open the valve and the pump will continue to transfer fluid. One can alternatively simply shut the pneumatic feed to the air motor of the pump and the pump will stop, open the valve and the pump will continue to operate. This is one of the simplest types of pumps to automate and integrate into a process.
4. Air Driven Diaphragm Pumps are also extremely popular where the process is to be operated by unskilled or semi-skilled labor. The safety aspect of both staff, equipment, process, and environment is a key factor in selecting this type of pump technology. Because the pump is seal-less, it will not leak even when it runs out of fluid to pump or when an operator closes the discharge valve in error. Mine or construction site dewatering applications are prime examples for this type of pump design.
5. Flammable environments or applications where the pump system is required to transfer flammable fluids, makes the intrinsically safe air operated diaphragm pump a unique and preferred technology.

The first step is to correctly identify what it is that you are pumping?

By clearly stating what one is pumping, the pump supplier will be able to assist with the correct material the pump needs to be constructed from in order to provide a long service life. Incorrect material selection will result in expensive repairs, downtime and possibly even safety issues for the operator.

The temperature of the fluid being pumped is also very important for the selection of the material of the pump casing, seals, and diaphragms. An incorrect specification in terms of the pumping temperature can result in the incorrect material choices being made. Premature pump failures with potentially dangerous consequences can at times be attributed to selecting a material that is not suited to the fluid temperatures being pumped

The easiest way to think about viscosity is to consider how easily a fluid flows. Honey for example, when cold does not flow really fast, heat it up and it will run like water. The viscosity of the honey is high when cold and low when warm or hot. It is important to know the viscosity of the fluid being pumped. Before we look for a pump, we must know the viscosity of the fluid being pumped. There is a viscosity correction table that we have developed, it is an aid in adjusting the pump curve for pump model selection. One must derate the capability of the pump because all pump performance curves are base on pumping water at around 20 Degree C. Obviously the pump will not perform to the same standards of the pump curve if it is to transfer a highly viscous, sticky, wood glue.

What volume of fluid needs to be transferred and how fast? A 200-litre drum of fluid needing to be decanted into a process reactor may need to be accomplished in just two minutes. That means a pump with a flow rate of 100 litres per minute must be selected. On the other hand, if the 200-litre drum is in a bulk break operation where the operator is filling 500 ml bottles, the practical flow of possibly 10 seconds may be required, this translates into a flow of 1 litre per minute. We need to clearly state the flow rate of the pump we need to select.

When we consider how the pump will be installed or used, there is obviously a few basic considerations, for example, the suction side of the pump, where will the pump draw fluid from?

Will the pump be standing on the base of a tank with the fluid highest level way above the pump?

If we have this type of configuration, where the pump is piped directly from the bottom of the tank, as soon as we open the tank valve, the fluid would flood the pump and it will be primed. In the pump industry, this is called a flooded suction. 

On the other hand, the pump may be located above the fluid level, so, for example, the pump is standing on a floor and the suction hose or pipe will then go down below the pump into a sump or basement. The pump will be required to draw the water from below it, this is called self-priming.

In both instances, the suction head is derived from the highest/lowest fluid point above or below the pump.

One should always take the worst case situation and design the pump to address that condition.

If the pump has to draw from a sump and the worst case depth of the water level below the pump is 6 meters, then that is a 6-meter negative suction head.

If the pump is mounted on the floor at the base of a 4-meter high tank and the minimum level in that tank is 1 meter above the pump suction, then that is your design positive suction head.

The discharge head is made up of the static head and the friction head.

Static head is the highest vertical point to which the pump needs to “push” the fluid in order for it to be delivered at the flow rate required. 

Friction head is the resistance of the fluid when driven by the pump through the pipe system.

This resistance is converted into an equivalent measurement in meters.

The internal diameter of the pipe, the length of the pipe, the number of bends and other components in the pipe system, chillers, heaters, valves, flow meters and filters must all be taken into consideration when working out the pressure that the fluid will need to be driven at in order to provide the desired flow at the end of the line.

1) Fluid Product A with Diesel base
2) Temperature 25 Degree C
3) Viscosity 1000 cps
4) Flow rate 120 litres per minute
5) Head Suction flooded 2 meters positive
Discharge static 5 meters
Friction 15 meters
Total 18 meters Or 1,8 Bar
Selection FLUIMAC MODEL P160, Aluminium Pump Body with Hytrel Diaphragms and Stainless Steel valves.

At 1000 cps the pump will not perform exactly as per the published pump curve and must be derated by 10%, According to the viscosity correction curve, this pump will only perform to 90% of its published curve.
If we select the P160, it will actually be performing at around 132 litres per minute on the curve in order for the pump to actually be producing 120 litres per minute in real terms.
Furthermore, the pump will need to operate at 7 bar air pressure in order to achieve this flowrate at the design pressure.