Notebook 5 : What other installation issues need to be considered?
Before going into other installation issues lets consider the requirements for designing the valves required for a leading dosing or metering pump. The parts of an intake or outlet valve consist of the valve seat, guide, stop, and static seal. Each can take many forms depending on the conditions for which the check valve is designed.
Valve slippage is the amount of pumpage that either leaks through the valve when it is closing or flows through the valve when it should be closed and is actually open. A perfect valve will close when the pump crank reaches 180o of the stroke and closes completely.
This is zero slip, however ball bounce, rimming, after the ball first reaches the seat as well as the time it takes the ball to reach the seat results in the valve actually closing some time after the crank reaches 180o and is the valve slippage.
Slip is influenced by pressure, speed, viscosity, spring loading, weight of valve’s moving parts and the quality of the machined parts tolerances. For example in high pressure situations poor machining tolerances or loose guiding will permit a check valve to contact off dead-centre. This contact causes a circling effect, which delays valve closure.
This “rimming” contributes to valve slip and can be extremely high in a poor quality dosing pump.
There are many ways of reducing slip and leading dosing pump manufacturers have adopted them to guarantee high performance from their pumps. Double-check valves is just one example of this development and the pumped fluid analysis is a necessary consideration when selecting the type of check valve to be used.
Not all check valves are ball type, poppet and cone type as well as disc and pinch are also used. However the ball check valve has as it’s most desirable feature the fact that it will turn each time it opens providing a different seating surface each time it arriving back on its seat, prolonging life and providing a self-cleaning action. Soft and hard seats are also selected depending on fluid to be pumped such as slurries, abrasive particles, or liquids that tend to coat out on the seat or valve.
Ball Check Valve
To get back to installation requirements the last comment was to do with fitting a injection valve or check valve at the dosage point to prevent unwanted mixing in the dosing line.
“What happens if there is a partial vacuum in the process line?”
A different problem arises if the process line is under slight vacuum as this will draw the dosage into the process line and all accuracy will be lost. To overcome this problem a pressure maintaining or pressure keeping valve is installed in the output line at the dosage point. Set at a pressure suitable to maintain correct dosing pump operation [nominally 1.5 bar] this will prevent the dosed fluid from being sucked into the process line and will also provide an ideal pressure situation for the dosing pump to do its task accurately.
Installation of a pressure keeping valve
“What happens of my dosing fluid supply tank runs dry?”
There is no danger to the dosing or metering pump as they are able to run dry for any length of time. However it is something to be considered when dosing into a pressurised process line or against a pressure keeping valve as air bubbles in the dosing line could prevent the dosing pump from delivering.
If this is a problem then a level switch or suction lance should be fitted to the intake side of the dosing pump, which will stop the pump when the dosage fluid container gets too low.
Consideration can also be given to the installation of an air-vent valve on the output side of the dosing pump . This will act as a suction aid after the tank has been filled by bleeding all the air from the pressure line.
How to avoid emptying of the suction line
“Where should I position my dosing or metering pump?”
As a dosing pump is a positive displacement pump and capable of delivering at high pressure it should be located as close to the dosing fluid supply source as possible. The suction line should be kept short and thin-walled tubing should be avoided. Generally dosing or metering pumps will have a 3m WC suction rating [although higher suctions are readily available] and any form of restriction should be avoided. To avoid the suction line from running empty on each pressure stroke when drawing through the top of the container, and the pump is higher than the max. liquid level in the feed tank, it is advisable to fit a foot valve.
This may also have a strainer incorporated into the valve. Suction lance design provides foot valve and level switch all in one.
When connecting into the side of the supply tank do so slightly above the tank bottom [nominally 100mm] and a dirt trap is advisable as sludge and dirt will result in dosing pump failure.
In the case where the feed is from a pressurised source, the installation of a “break-tank” which consists of a tank fitted with a ball float-valve, will ensure that pressure conditions on the suction side are neutralised.
Installation of a line strainer
“Are there any other supply-side installation considerations?”
It is possible that the supply tank is deep and for technical reasons cannot be accessed through the side. Using a siphon pipe fitted with a foot valve and either a filling device or a siphon tank and hand pump, the suction line can be filled and all the air removed prior to starting the dosing pump.
However, acceleration pressures might have to be taken into consideration, due to the increased length hence capacity of the suction line, and possibly requires the installation of a pulsation damper. This type of installation may be a legal requirement where spillage is not allowed due to possible contamination of ground water etc.
Use of siphon system
When piping up the dosing or metering pump for the handling of low vapour-pressure fluids a flooded suction is recommended, and if possible a slight gradient in the pipe sloping away from the tank will allow any gas formed to vent back into the tank
Slight down gradient from tank
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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