Air Operated Double Diaphragm Pumps Guide

Air Operated Double Diaphragm Pumps Guide

How to Read a Performance Curve

How to Read a Performance

All performance curves are based on water at 20°C with the pump having ample NPSHa. Other circumstances, such as changing in viscosity and suction lift, will change the performance.

In the above example, a flow rate of 25 l/min is desired and a discharge head of 30 m has been calculated.

A line is drawn up from the desired flow rate on the bottom axis and a separate line drawn across from the desired head on the left axis. Where they intersect provides an indication of the required air pressure at duty point (Dark Blue line) as well as the air consumption at duty point (Light Blue line).

In the above example, when considering the T50, the pump would require an air pressure of 5 bar and would consume approx 0.2 Nm³ air per minute. If the customer doesn’t have this amount of air pressure and volume available from their air supply then it would be advisable to increase the size of the pump to the T100, which for the same duty would only require approx. 4 bar air pressure and only consume just over 0.1 Nm³ air per minute.

 

Changes in Capacity

As mentioned, an AODDs capacity will vary according to changes in both viscosity and suction lift. These variations need to be taken into account when selecting a suitable pump.

Below is a chart displaying the % flow rate available according to changes in suction lift.

Changes in Capacity

If the intended application requires a 4 m suction lift, for example, then the % of available flow rate from the pump in question would only be 80%. Therefore, when considering a pump for a suction lift application you need to add onto the flow on the curve the % drop in flow to achieve the required flow rate. In this example, it would be 20%. So when considering the T50 for 25 l/min @ 30 m Head, with a 4 m suction lift, you would need to plot 31.25 l/min from the flow axis to determine the required air pressure and air consumption.

In the case of the T50, this would change the air pressure required to 6 bar and the air consumption to approx. 0.26 Nm³. The pump would be capable of the application however would be working hard, consume a lot of air and be susceptible to increased wear and tear and mechanical failure if used 24/7.

Please also note that, as mentioned, dry suction lift capabilities change according to the size of the pump as well. The T50 is only capable of 2.5 m max dry lift (when fitted with SS balls) and the T100 capable of 3.5 m. Therefore both pumps would need manually priming on installation as they wouldn’t be able to prime themselves. The dry suction lift capabilities of each pump size are available from the website, pump data sheets and within the product brochures.

Similarly, when considering pumping a viscous fluid the % flow available from the pump also changes and needs to be taken into account when selecting an AODD.

Below is a chart displaying the % flow available according to changes in viscosity.

Viscosity

If the intended application requires pumping a fluid with a viscosity of 3000 cPs (Shampoo like), for example, then the % of available flow rate from the pump in question would only be 80%. Therefore, when considering the T50 for a required capacity of 25 /min @ 30 m Head, with a 4 m suction lift, pumping a fluid with a viscosity of 3000 cPs, you would need to add 20% onto the duty flow for the suction lift and an additional 20% for the viscosity correction. Taking your plotted flow rate on the curve to 40 l/min.

In the case of the T50, this would change the air pressure required to just under 8 bar and the air consumption to approx. 0.4 Nm³. This would render the T50 unsuitable for the intended application as not many site have this amount of air pressure and volume available and you would need to consider either the T100 or even T200.

 

Other Considerations

Chemical Compatibility: of the wetted components (housing, diaphragms, balls & seats). Verifications need to be made to ensure that the pump is capable of handling the fluid in question. Charts and spreadsheets & web based tools are available for verification purposes.

Solids Handling Capability: if the pump is intended to handle any solids in suspension then consideration needs to be taken with regards to the max size of solids the pump is capable of passing. This info is also available from the website, pump data sheets and within the product brochures. In the case of the Sanitary pumps, we can fit Flap Valves, instead of the standard Ball Check Valves, when pumping liquids containing big solids without damage. We are able to pump solids up to 50 mm in both T225 and T425 pump sizes and an impressive 100 mm in the T825 4” pump. Pumps can reach dry suction lift of 4.5 meters.

Suction & Discharge Line Sizes: It is best practice to have the suction and discharge ports the same size or 1 size smaller than the installation pipework. This is particularly important for the suction side as if the pipe is smaller than the pump ports and there is a decent suction lift or distance between the fluid source and the pump then the pump may be starved.

Pulsing Flow: As AODDs create a pulsing flow by virtue of their design consideration needs to be taken for this fact when evaluating the suitability for the intended application. If the customer requires a near constant flow (metering applications for example) then we would need to also install a Pulsation Dampener either fitted to the pump or the pipework. Alternatively, if the customer has a 24V DC power supply available at the point of use then they could also consider a LEAP unit without a Pulsation Dampener, reducing the overall purchase cost and complexity of the system.

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