Pressure Independent Valves Function
The EVOPICV axial pressure independent control valve consists of three main functional groups:
- Differential pressure regulator
- Oblique globe valve for temperature control
- Movable seat for flow rate adjustment.
1. Differential Pressure Regulator
The differential pressure regulator is the heart of the pressure independent control valve, by keeping a constant differential pressure across the valve seats constant flow and full authority temperature control can be achieved.
Incoming pressure P1 is transmitted to the top face of the diaphragm, outgoing pressure P3 is transmitted to the underside of this same diaphragm. A constant effective differential pressure is maintained between P2 and P3. As P1 increases relative to P3 it acts on the diaphragm closing the shutter (A) against a seat (B) thereby lowering the effective differential pressure.
As P1 decreases relative to P3 the diaphragm acts to open the shutter (A) from the seat (B) thus increasing the effective differential pressure. The diaphragm acts against a spring in order to balance the pressure control and stop the diaphragm oscillating.
Water flow through a valve varies as a function of the area of passage and the pressure differential across that valve. Due to the incorporation of the differential pressure regulator the differential across the valve seats P2 – P3 is constant meaning that flow is now only a function of area of passage.
2. Temperature Control Valve
The temperature control element of the valve consists of an oblique pattern globe valve the differential pressure (P2-P3) across which is held constant by the differential pressure regulator.
The authority (n) of a valve can be calculated from the pressure drops across that valve compared with the local system. In this case written as
In the case of a Pressure Independent Control Valve the system Pb is close to 0 meaning that the authority is very close to 1.
3. Maximum Flow Rate Adjustment
In order to preset the maximum flow rate of the valve a second seat is used, again the differential pressure across this seat is held constant (P2-P3). The area of passage is changed by moving the hand wheel causing a profiled disc to move against a fixed seat thereby adjusting the flow.
Figure A. describes the general flow performance of the valve as the differential pressure changes.
It can be seen that before the start-up pressure is achieved the flow rate increases almost as a fixed orifice valve. Once the start-up pressure has been achieved the valve controls the flow within the set point range.
Values stated for the start-up pressure are calculated with the valve in the fully open position as the lowest differential pressure at which the valve will give a constant flow (±5% of nominal). It can be observed that as the valve is preset to lower flow rates that the start-up pressure de-creases.
It can also be seen that once the working differential pressure range has been exceeded that the flow rate begins to rise out of the tolerance bands, however this happens at a much lower rate than a fixed orifice valve would exhibit.
It should be noted that for a particular pressure a range of flows (within ± 5% of the nominal) can be produced depending on if the pressure is rising or falling. This hysteresis effect is typical of dy-namic balancing valves due to the internal tolerances of the pressure regulator.
When designing the pipe system the start-up pressure should be used as the nominal resistance of the valve for pump sizing purposes.
The valve characteristic is a measure of the rate at which the valve controls flow in relation to its opening position, authority is a measure of how well a valve performs in relation to its characteris-tic curve when in use.
Power output through a coil is related to water flow rate but it can be seen from figure a. that this relation ship is not linear. It can be seen from figure A. that as flow increases the power output tends towards some maximum value. It can also be seen that power output increases rapidly from 0 - 50% of water flow and thereafter the rate of increase of power output decreases. The steep-ness of this curve is typically dependant on the temperature difference induced in the heating or cooling media (ΔT).
Valve Characterisation Curves
Figure B. describes an ON/OFF or quick acting valve characteristic, it can be seen that flow rate increases rapidly until 30% of the valve stroke and then slowly thereafter.
Figure C. describes a Linear valve characteristic, flow rate increases in direct linear proportion to the valve stroke.
Figure D. describes an Equal Percentage (modified logarithmic) valve characteristic. It can be seen that flow rate increases slowly until the valve stroke is approximately 70% and thereafter flow rate increases rapidly.
It is generally desirable that the power output of a coil is linear in relation to the valve stoke as this results in the most easily controllable situation.
System Response Curves
Figure E. describes the system response (power output of a coil vs. valve stroke) when a quick act-ing or ON/OFF valve is used. It can be observed that the power output rises to over 95% before the valve is more than 20% open.
Figure F. describes the system response when a linear valve is used, it can be seen that power out-put rises quickly for the first 50% of valve stroke and thereafter the rate of change decreases. It can also be seen that 95% of the power output is achieved with a valve stroke of approximately 80%.
Figure G. describes the system response when an equal percentage valve is used, it can be observed that power output increases linearly with increasing valve stroke.
From the above graphs it can be seen that the equal percentage characteristic gives the most desir-able system response. It should be noted that in any practical system the valve characteristic and coil characteristics may not be 100% matched but an equal characteristic valve will always give a more linear response than any other profile.
