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How to select a
suitable solenoid valve?
In selecting a solenoid valve, a number of operating and physical
parameters must be considered. The operating parameters include opening
pressure, flow, pressure drop, temperature and maximum pressure the solenoid
valve may experience. The physical parameters are pipe size and type of fluid.
1.
PRESSURES
MAXIMUM OPERATING PRESSURE
DIFFERENTIAL –MOPD (PS)
When the solenoid valve is closed, the supply pressure is present at
the inlet port only. This is the pressure the solenoid valve has to open
against. In other words, this is the pressure the electrical solenoid has to
overcome to open the solenoid valve and allows flow to occur. This pressure is
called Maximum Operating Pressure Differential (PS) or simply “MOPD” in our
catalogue. The value given in the catalogue must be equal to or greater than
the maximum pressure at the supply port at which the solenoid valve must open.
Note it is not always the same for different types of fluid and that AC solenoid
valves usually have higher pressure ratings than DC solenoid valves.
Strictly speaking, the MOPD is the maximum pressure drop across the solenoid
valve when the solenoid valve is closed. If there is a pressure at the outlet
port when the solenoid valve has to open, this could be subtracted from the
inlet pressure to arrive at the MOPD. However , if at some time a zero pressureis present at the outlet, the solenoid valve will have to open at the pressure
at the inlet which may be too high, causing possible coil burnout on an AC
source. Therefore , the supply pressure is considered as the MOPD in
conservative designs.
MINIMUM OEPRATING PRESSURE
DIFFERENTIAL
Once the solenoid valve has opened, the pressure conditions may change.
A pressure is present at the solenoid valve outlet. This pressure may be
negligible as when the solenoid valve is filling a tank, or it could be quite
high when the solenoid valve supplies a spray nozzle. In some cases, the
pressure at the supply and outlet will be the same when a cylinder has moved
and we have in effect a dead-end load. When the pressures are equal, the fluid
has ceased to flow.
If we call the supply port pressure P1 and the outlet port pressure P2,
The pressure drop across the solenoid valve is P1-P2.We also call this the
pressure differential (△P)across the solenoid
valve.
Example: if P1 was 10 bar and P2 was practically
zero, which would be the case if water was flowing out of the solenoid valve
into a tank, the pressure drop or differential would be △P=P1-P2; that is
10-0=10 bar.
Now: if P2 was 9bar due to a nozzle
△P
would equal 10-9=1 bar
(The minimum operating pressure differential is the minimum pressure
drop that will exist across the solenoid valve when there is flow).
In selecting a solenoid valve, check if a minimum operating pressure differential
column is applicable to the solenoid valve you are considering. If there is no
column or the number in the column is zero, the solenoid valve will remain open
even if the pressures at the inlet and outlet are the same. If there is such a
column, make sure that the minimum pressure drop in the system never gets below
the figure shown or the solenoid valve will throttle , causing fluctuation in
flow.
MAX.ALLOWABLE PRESSURE
(According to EN764)
The maximum allowable pressure is the maximum line or system pressure
the solenoid valve can be subjected to in normal service. Due to limitations on
the maximum pressure test we can conduct on our production test stands, the
figures shown may be conservative. If higher safe working pressures are
required, Pls. contact with us.
Pressures up to the maximum allowable pressure can occur when the solenoid
valve is open or closed and may damage the solenoid valve internal parts if
exceeding the MOPD.
PROOF PRESSURE
2.
TEMPERATURES
Normal ambient temperature
The normal ambient temperature is
assumed to be in accordance with standard conditions as specified in ISO 554
Ambient temperature: 20 ℃
Ambient pressure: 1013mbar
Relative humidity: 65%
Maximum ambient temperature (TS)
The
maximum ambient temperature listed (20 ℃) is
based on test conditions to determine safe limits for coil insulation. The
temperature is determined under continuously energized conditions and with
maximum fluid temperatures (as listed) existing in the solenoid valve. In many
applications the existing specific conditions will permit use at higher ambient
temperatures. In addition, modifications to standard constructions are
available which can extend the maximum ambient temperature limitation to 80℃ or
more.
Minimum ambient temperature
The minimum ambient temperature of a solenoid
valve is greatly affected by application and construction.
Damage may occur when liquids solidify above the specified minimum temperature.
Special constructions are available
for lower temperature, Pls. contact us freely.
Maximum fluid temperature (TS)
The maximum fluid temperature listed
is valid for an ambient temperature of 20 ℃and 100%
RD (Relative Duty Time). In case of higher fluid temperature, refer to
3.
VISCOSITY
Viscosity is the resistance of a fluid
to flow, due to internal friction. Viscosity affects the flow rate of a solenoid
valve considerably and the flow factor is reduced when viscous fluids are to
pass the solenoid valve.
There are two types of viscosity:
a) dynamic viscosity, expressed in Pa.s
(Pascal seconds) or Poises
b) kinematic viscosity, which is the
ratio between dynamic viscosity and density of the fluid
4.
RESPONSE TIMES:
This is the time-lapse after energizing
(or de-energizing) a solenoid valve until the outlet pressure reaches a
specific percentage of its maximum steady value, the outlet being connected to
a circuit having specified flow parameters.
Response time depend on 5 factors:
- Kind of
electrical supply: AC or DC
- Fluid
handled by the solenoid valve, viscosity and pressure level
- Type of
operation: direct or pilot operated
- Size of
the moving parts of the solenoid valve mechanism
- Circuit
in which the time is measured
Approximate values for AC solenoid
valves on air service under average conditions are:
a) small direct acting solenoid valves: 5 – 25ms
b) large direct acting solenoid valves: 20 – 40ms
c) internal pilot operated solenoid valves:
1) small diaphragm
type: 15 - 60ms
2) large diaphragm type: 40 - 120ms
3) small piston type: 75 - 100ms
4) large piston type: 100 - 1000ms
Generally speaking, operation on
liquid media will have the following effects:
a) small direct acting solenoid valves ±20-30% higher
b) large direct acting and internally pilot operated solenoid
valves: dependent on size 50-150% higher
Response time on DC operation is approximately 60% higher
than on AC operation.
Consult us if response time is a critical factor.
The ratings below are associated with the speed of
response of self-contained pilot operated solenoid valves, using the flowing
stream as the operating
Excellent: 0 to 500 SSU
Good: 500 to 1000 SSU
Satisfactory: 1000 to 2000 SSU
Fair: ; ; 2000 to 5000 SSU
Unsatisfactory: above 5000 SSU
Normally a solenoid valve application that falls into the
fair category performs or responses very slow with regulating pilots such as
pressure reducing, back pressure or pressure relief. A solenoid on-off solenoid
valve will perform satisfactorily provided an overrun can be compensated for.
Electrically
operated directional control solenoid valves
The switching time of an electrically operated
directional control solenoid valve is the time which elapses between the
closing or opening of the electric circuit and the instant when the pressure at
the outlet port reached 50% of its maximum value , the outlet port being
blocked at the solenoid valve body or at the sub-plate if the solenoid valve is
mounted on a sub-plate.
Electric (direct
operated)
The switching threshold of a direct electrically operated
solenoid valve is the limiting value of the electric voltage, rising or falling,
which causes or allows “switching”, i.e . the complete transition from an
initial to a final state under the normal operating conditions foreseen by the
manufacturer (no oscillation, correct sealing,normal flow values, etc).
Electric (pilot
operated)
Here consider only the case where the medium causing
switching is compressed air coming either from the main inlet port or from an
external auxiliary port.
The switching threshold of an electric pilot operated solenoid
valve is the joint effect of two limiting values , rising or falling ;on the
one hand the signal pressure and on the other the signal voltage , which cause
or allow “switching” , i.e. the complete transition from an initial to a final
state under the normal operating conditions foreseen by the manufacturer (no
oscillation , correct sealing, normal flow values etc.).
SOLENOID
VALVE SEAT TIGHTNESS:
Solenoid valve seat tightness or leakage depends on the
type of solenoid valve, used sealing materials trim and medium.
Although it is expected that a big orifice piston solenoid
valve with high DURO disc will show some more leakage flow than a simple
core-disc execution with soft resilient. To establish practical test leakage
flow rates for the solenoid valve the following 3 groups(categories) are
applicable for all solenoid valve types and / or sizes.
1) Leakage flow <0.24N dm3/h within the specified
pressure range of the solenoid valve product (all resilient disc, diaphragm or
poppet closing members such as NBR, FPM, EPDM, TPE, UR, etc.).
2) Leaking flow <0.084N m3/h within the specified
pressure range of the solenoid valve product (all non-resilient disc or other
closing members such as PTFE, reinforced PTFE, metal, POM, etc.).
3) Leakage flow for “Gas Approved solenoid valves” see
table below
DEGREES OF PROTECTION PROVIDED BY ELECTRICAL ENCLOSURES (
IP CODE)
(according to standard EN60529 and IEC529)
The code letters IP (Ingress Protection) followed by 2
characteristic numerals: e.g. IP65.The first figure indicates the degree of
protection of the energized parts and internal moving parts against ingress of
solid foreign objects. The second figure indicates the degree of protection
against ingress of water with harmful effects.