Mobile Vs Fixed Installations

in Diaphragm Pumps

The operation and longevity of air operated diaphragm pumps are often affected by the style and operation of the pump unit.

When purchasing a diaphragm pump unit, consideration needs to be given to the way this unit will be operated in order to not only enjoy a troublefree ongoing operation without continual stoppage and downtime frustrations, more importantly, to protect the operating budget.

Maintenence/ repairs and the associated costs can be greatly reduced by the correct installation of auxiliary components that support the actual pump unit.


Double diaphragm pumps by their very design, transfer fluid from the diaphragm chamber by exerting air pressure on the air side of the diaphragm which is greater than the pressure the fluid side will exert on the diaphragm. The higher pressure on the air side caused the diaphragm to expand into the fluid chamber, thus the diaphragm forces the fluid out of the liquid side of the pump chamber. 

In a fixed installation, the liquid pressure is relatively fixed, in other words, pipe lengths are stable, filters, heater or cooling coils, valves and other fluid side losses remain relatively constant. This also means the air pressure regulator can be fixed and left at a specific setting. In other words, the air pressure is fixed and balanced to generate a specific force on the diaphragm which will generate the pump flow rate required by the system.

Every time the pump starts, it starts against a fluid side of the diaphragm which is full of liquid with a known pressure. The pump produces the required flow based on the air pressure as set on the air pressure regulator when commissioning the system.

The operator or plant automation simply turn on the air side valve or the fluid side valve to start or stop the pump. There is no need to reconsider the pump system as it is a static installation. The only time this changes is when the discharge pipework is cleared, emptied for or when a new component needs to be installed.  This is the added challenge to operating air diaphragm pumps in a mobile installation.

A mobile installation, however, requires an operator that understands that the pump, having been moved from the store or another location on the plant to this new pump position, will now start with an empty pump chamber. An empty suction hose or pipe (in a self-priming situation) and an empty discharge line. If there is a filter regulator attached to this pump, it is likely that the regulator will have the setting of the previous pumping position. It may be as high as plant pressure up to 8 bar.

Imagine what happens when the plant air is applied for the first time to this diaphragm. Zero fluid in the fluid side of the chamber to counter the 8 bar on the air side. The result is that the diaphragm goes into overdrive, “searching” for a balance to the air side pressure. One will often hear the pump cycling (oscillating from one side to the other) at a fast pulse rate until fluid enters the chamber and the discharge line begins to fill. As the line fills, pressure begins to equalise and the pump slows to a more steady pace. If the diaphragm installed is Teflon, it is very likely to already have stretched and possibly even cracked or in other ways been damaged by this action.

The two installations, fixed or mobile, while similar, need a very different operator and management. The auxiliary equipment fitted needs to be the same for both installations, however, the operator needs to be trained and far more aware of the “commissioning” of this pump in its new position.

Basic auxiliary equipment:- 

  • A baseplate or wheeled trolley.
  • An air filter regulator.
  • A fluid side suction strainer. (if required to protect the pump from debris)

Correct commissioning of a diaphragm pump from an air pressure point of view.

  1. Adjust the plant pressure at or as close to the pump as possible with the individually installed air filter/regulator to around 2,5 Bar.
  2. Turn on the plant air to the pump slowly adjusting the volume of air with the air valve so that the pump cycles at around 60 to 70 beats per minute.
  3. If the pump starts to slow, slowly open the valve for the plant air line. 
  4. If the pump is still slower than 60 to 70 “beats” per minute slowly open the air pressure regulator to increase the air pressure onto the diaphragms.
  5. As the pump fills the discharge line and starts to take up the back pressure in the pipeline, the balance pressure demand on the diaphragms will increase.
  6. Once the pump is cycling at 60 to 70 cycles per minute, stop increasing the air pressure and lock the filter/regulator in place, (normally pushing down on the adjustment cover)

The above commissioning is normally only done once in a fixed installation, however, in a mobile operation where pumps are moved from application to application, this procedure needs to be done every time the pump is moved and started.

 

 

 

How to select a diaphragm pump?

How to select a diaphragm pump?
in Diaphragm Pumps

Why 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.

First step in selecting a Diaphragm pump

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.

Material Choices For Diaphragm Pumps

Secondly, The Pumping Temperature?

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

Thirdly, know the Viscosity?

Viscous fluid flowing from container
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.
Diaphragm Pumps Viscosity Correcion Table

Fourth, Flow Rate?

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.
Pump Performance Curve for selecting flowrate

Fifth, The Pump Head or Operating Pressure?

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.

How to Select your Diaphragm Pump

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.