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Description of a pulsation damper and how it works
A pulsation damper is a vessel with gas inside, normally NITROGEN.
The initial filling or inflating gas pressure inside the dampener is always lower than the circuit pressure.
The inflating gas pressure will be called “Po”.
In all pulsation dampers there is an element to isolate the gas from the circuit liquid; its main function is to avoid the gas loss. This piece that separates both fluids is made basically with two materials:
With rubber – Nitrile, EPDM, FPM, Butyl, Silicone, etc and thermoplastic usually PTFE.
When the rubber is used the dampener is defined as bladder or bag type and if the PTFE is used the dampener can be membrane or bellows type according to the separator element form.
The use of one type or another to separate material and form-bladder, membrane or bellows- will depends generally on the special performances of the circuit such as: the pressure, temperature and the possible corrosive effect that could be produced by the circuit liquid.
The performance of a pulsation damper in a hydraulic circuit with a dosing or metering pump-with piston or membrane- is to stabilise the variable and oscillating flow in each revolution of such type of pumps.( we will see later the characteristics of this types of pumps) The main performance of this pumps is to deliver a constant volume of liquid during one complete revolution with independence of the circuit resistance or pressure.
When a pulsation damper has been installed the volume supplied by the pump during a complete rotation or work cycle is divided in two parts; one is going to the circuit needs and the other part goes into the pulsation dampener. This volume stored into the dampener is returned immediately to the circuit while the pump is in its suction cycle.
To the amount of liquid going into and out of the dampener in each cycle or pump revolution we will call “dv”.
When “dv” is introduced into the dampener the gas filled inside will reduce its volume and increase its pressure, the final gas volume plus the volume of liquid introduced will be equal to the initial gas volume.
The initial gas volume is the total dampener volume or the dampener size. The dampener size is the unknown value to calculate and that will depend in all cases on the pump performances. To the dampener size we will call “Vo”
We can establish that : V2 + dv = Vo, ( V2 is the final gas volume)
Each dampener has a constant which depends on the charging gas value and its size ; Po x Vo = constant.
When the dampeners are working is not convenient that all the liquid stored goes out in each cycle keeping the dampener empty of liquid, this will damage prematurely the bladder or the membrane when the insert fixed on it is hammered against the dampener internal bottom.
We will have a new formula : V2 + dv + v = Vo
Where “v” is a non used volume of liquid inside the dampener; as a norm this volume is the 10% of the total dampener volume the former formula will change to:
V2 + dv + 0,1Vo = Vo; and from this Vo = ( V2 + dv ) / 0,9
The following graph and the figure representing the three states of gas volume inside the dampeners will made more clear everything exposed above.
This graphs shows ;
At charging gas value “Po” there is no liquid inside the dampener. The curve cuts the ordinate axis in the point where corresponds a zero value in the abscissa axis.
The pressure “P1” is the gas pressure when the volume “v” has been introduced into the dampener; the pressure “P2” is the value reached by the gas when the additional volume “dv” is into the dampener .
From this curve we can see that for a fixed dampener size if the value “dv” increases then the pressure value “P2” will also increase, or if we increase the dampener size keeping constant the value “dv” the final pressure gas value “P2” will be lower.
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