Notebook 4 : What does a complete dosing or metering pump installation consist of?
Taking a general application where an additive is to be dosed from its bulk container or IBC into a process continuously and without pulsation. No automatic control is required as manual stroke adjustment is sufficient to attain the level of dosage required. Accurate calibration of the dosage rate is however a pre-requisite and pump protection is required to protect against pump overload should the main process plant shut down unexpectedly and the dosing pump continue to run.
Incidentally diaphragm pumps can run dry but will self-destruct of they pump against a closed head so fit a pressure relief valve.
Installation with (external) relief valve
Details of the additive have been received and the pump materials selected in this case pump head and valves are Polypropylene with Viton 0-rings and PTFE-faced diaphragm. Capacity has been sized for roughly 70% normal operation and because of the size of the pump an external pressure relief valve has been specified for pump protection as integral pressure relief is not available on a pump of this size.
Pressure relief is set for 3 bar and pump delivery head under normal process conditions is calculated to be close to 2.5 bar. The supply tank is checked to ensure it is covered and not subject to dirt ingress and that the outlet to the dosing pump is above the floor of the tank [+-100mm] to avoid any sludge or sediment being drawn into the pump suction. After an isolation valve fitted to the tank, which should also have a drain valve, a dirt trap should be considered if the solution is prone to sedimentation then install the calibration cylinder close to the pump inlet or suction valve.
This is fitted with an isolation valve and for calibration purposes it is filled by opening this valve and allowing the level to rise under head pressure from the supply tank, until sufficient fluid is in the cylinder. The supply tank is now isolated and the dosing pump started and allowed to dose drawing out of the calibration cylinder until a predetermined time has lapsed. This is then checked against the dosage taken up and used to adjust the pump manual stroke adjustment until the required capacity or dosage rate is achieved.
A pulsation damper is installed in the dosing pump output or delivery line and this will smooth the pump delivery to ensure a continuous accurate flow. At the point of addition into the main process a non-return valve is fitted to prevent back flow from the main line.
Installation of injection valve
“What happens if I want to dose into an open tank and it is positioned below the additive tank and dosing pump?”
Dosing pumps are designed with a two stage action [see Note Book No.1] and require internal pressure to get the intake and output valves to operate. If the point of delivery is lower than the supply side the fluid being dosed will siphon through the pump and the pump becomes inaccurate.
To prevent this or excess delivery it is necessary to provide a back pressure on the output line. This can be done by repositioning the supply tank and dosing pump to below the point of delivery.
Alternatively, and more commonly, a pressure keeping or pressure retaining valve is installed in the output or delivery line usually set at a nominal relief pressure [1.5 bar] and in both cases this provides sufficient head on the pump output or delivery to allow the input and output valves to operate correctly. Later on we will discuss “valve slippage” and how this affects valve design.
Installation of a pressure keeping valve
“What steps should be taken to protect the dosing pump because of it’s positive displacement design?”
Being positive displacement dosing pumps must be protected from damage due to over pressure as a result of blockage in the delivery line or the closure of a down stream valve, etc. This is best achieved by fitting a pressure relief valve into the system and this can be either integral in the dosing pump head or as a separate device. Smaller capacity dosing pumps are usually fitted with integral pressure relief valves and larger capacities tend to use separate pressure relief valves although there is no hard and fast rule. The important issue when installing and piping a pressure relief valve is to ensure that it is laid with a gradient that will allow it to drain into an open storage tank or discharge outlet. Alternatively it may be piped into the dosing pump suction line.
Installation of a pressure relief valve
“How do I prevent fluid from the main line from entering the dosing line?”
When dosing into a main line with an elevated process pressure it is important to prevent the main line fluid from entering the dosing line on each pulse as undesirable mixing will take place in the dosing line and could result in early failure of the system. This is overcome by fitting an injection valve or non-return valve at the dosage point.
Installation of injection valve
Notebook 1 : “What is a dosing or metering pump.?”
Notebook 2 : “How do I select a dosing pump?”
Notebook 3 : “How do I install a dosing or metering pump?”
Notebook 4 : “What does a complete dosing pump installation look like?”
Notebook 5 : Continues the discussion on liquid dosing technology.
If you have any questions please go to liquiddosing@dosetech.co.za
Pumps
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