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Solenoid Valves Selection Guidelines

What you must specify when selecting solenoid valves

  • Type of valve required (solenoid or pressure actuated)
  • Body and valve internal material(s)
  • Fluid flowing through the valve
  • Line pressure at the valve and allowable pressure drop (pilot operated or force lifting principle)
  • Nominal diameter required (matching process line size or from flow calculations)
  • Connection (NPT, G or flange)
  • Fluid temperature
  • Ambient temperature
  • Switching function (normally-open or normally-closed)
  • Available electrical power
  • Protection classification (IP rating and explosion-proof)

Line Pressure and Pressure Drop

Fluid pressure at the valve inlet and allowable pressure drop are very important parameters for selection of the valves. The piston valves are designed to withstand up to 600 psi (40 bar) pressure, while the diaphragm valves are suitable for up to 230 psi (16 bar) pressure -see the data sheets.

Pressure drop across a valve can be calculated when the valve sizing coefficient (Cv or Kv ) and the flow rate of the fluid along with its specific gravity and viscosity have been determined.

Valves with or without ∆P

When pressure drop is a concern, There are two groups of valves.

(1) The pilot operated solenoid valves, which require a minimum differential pressure for operation, and

(2) the force lifting solenoid valves that do not require a pressure differential to operate.

The pilot operated solenoid valves operate on the servo assistance principle, which requires a specified differential pressure for opening and closing. These valves have a pilot and bleed orifice which enables them to use line pressure for operation. In the normally-closed valves, when the solenoid is de-energized, the pilot orifice is closed and full line pressure is applied to the top of the diaphragm or piston through the bleed orifice, providing seating force for tight closure. Provided the differential pressure between the inlet and the outlet of the valve be at least equal to or greater than the required Δp, the valve would remain securely closed. The valve will only close tightly in the direction of flow. Flow in the opposite direction to the arrow may damage the valve. When the solenoid is energized, the pilot seat will open, the pressure on the main closure device will be relieved, and raised into the open position by the increasing effective force on the underside. The line pressure will keep the valve open.

The force lifting solenoid valves are designed for reliable service in the vacuum and low pressure ranges, where any differential pressure is insufficient to allow the use of servo assisted solenoid valves. The force produced by the solenoid plunger, which is mechanically coupled to the main closure device, opens this type of valve. The sequence starts with the solenoid opening the pilot seat. This relieves the pressure on the main closure device, bringing it into balance so the solenoid force can lift it into the open position. When the pilot seat is closed, bleed orifices allow a force to build up on the closure device that pushes it down into the closed position on the valve seat.

Actuation and Valve Types

The Solenoid valves are designed to control (on/off) the flow of fluids by the aid of a solenoid or pressure actuation. They are divided into two major categories: (1) Solenoid Valves and (2) Pressure Actuated Valves.

A solenoid valve consists of a valve and a solenoid (electro-magnet) which controls the valve. A pressure actuated valve, an angle seat valve with a pressure actuated system on top, to control the valve.

VALVE TYPES

Diaphragm Valves

  • Suitable for maximum operating pressure of 230 psi (16 bar)
  • Suitable for maximum viscosity of 25 cst
  • Good for operations with or without differential pressure
  • Valve body made from stainless steel or brass
  • Connection sizes between ¼” and 2″, NPT and G

Piston Valves

  • Suitable for maximum operating pressure of 600 psi (40 bar)
  • Suitable for maximum viscosity of 150 cst
  • Can be used for fluids up to +200ºC temperature
  • Good for operations with or without differential pressure
  • Valve body made from stainless steel or brass
  • Connection sizes between ¼” and 2″, NPT and G
  • Damped operation standard

Pressure Actuated Valves

  • Suitable for contaminated and very viscous fluids
  • Can be used for fluids up to +180ºC temperature
  • Low closing shock is the result of fluid flowing against the valve plate
  • Valve body made from stainless steel, cast iron, cast steel or gunmetal
  • Connection sizes between ¼” and 2″, NPT and G

Note: The above mentioned specifications may change from vendor to vendor.

Flow Rate Calculations

Valve models must be carefully selected and accurately sized to suit the system application. Once the permissible pressure drop across the valve have been determined, specific gravity and flow rate of the fluid govern the connection size (non-viscous fluids).

A basic sizing equation for liquids can be written as follows: Q(liquids) = Cv X SQ.RT(∆P/s.g.)

where

Q is the flow rate in U.S. gallon per minute.

Cv is the valve sizing coefficient determined experimentally, and is defined as the number of U.S. gallons of water at 60°F that will flow through the valve in one minute, when the pressure differential across the valve is 1 psi.

∆P is the pressure drop at the valve is 1 psi.

s.g.” is the specific gravity of the liquid (s.g. of water at 60°F is 1.0000).

Viscous conditions can result in significant sizing errors in using the above equation, because the published Cvvalues are based on test data using water as the flow medium.  Although the majority of valve applications will involve fluids where viscosity corrections are relatively small, fluid viscosity should be considered in each valve selection. By using an appropriate nomograph, the standard Cv coefficient can be corrected for viscous applications.

For gases the equation is: Q(gases) = 16.07Cv X SQ.RT{[z(p12-p22)]/[Tx(s.g.)]}     (non-critical flow)

where

Q is the gas flow in SCFM.

Z  is the compressibility factor.

p1 is the upstream pressure in psia.

p2 is the downstream pressure in psia.

T is the temperature in Rankin scale (°F + 460)

“s.g.” is the specific gravity of the gas (air = 1)

If the flow coefficient is given as Kv, the equation will be: Q(liquids) = Kv X SQ.RT(∆P/s.g.)

where

Q is the flow rate in m3/h

Kv is the valve sizing coefficient determined experimentally, and is defied as the number of cubic meter of water at 15°C that will flow through the valve in one minute, when the pressure differential across the valve is 1 bar.

∆P is the pressure drop at the valve is 1 bar.

“s.g.” for water between 5°C  and 30°C  can be assumed 1.

The flow coefficient tabulated for each valve allows calculations of parameters such as flow rate or pressure drop for steady-state flow.

For gases the equation is: Q(gases) = 341Kv X SQ.RT{[z(p12-p22)]/[Tx(s.g.)]}     (non-critical flow)

where

Q is the gas flow in SCMH.

Z  is the compressibility factor.

p1 is the upstream pressure in bara.

p2 is the downstream pressure in bara.

T is the temperature in Kelvin scale (°C + 273.15)

“s.g.” is the specific gravity of the gas (air = 1)

Fluid and Ambient Temperatures

In order to ensure that there is no thermal damage to the solenoid valve, the specifications for the maximum permitted fluid and ambient temperatures should not be exceeded.  The highest permissible valve temperature is generally determined by the thermal durability of the sealing materials. Temperature durability of important sealing materials used inside solenoid valves are specified as follows;

NBR(example: Buna “N”)-10…+90°C
CR(example: Neopren)-20…+90°C
EPDM(example: Nordel)-20…+130°C
HNBR(example: Therban)-20…+150°C
FPM(example: Viton)-10…+180°C
PTFE(example: Teflon)-20…+200°C
Kalrez(example: Perfluroide Elastomer)-30…+200°C

Temperature of the coil should also be checked for safe operation.  When in operation, the coil temerature is influenced by 3 factors:

– Temperature of the Fluid

– Ambient Temperature

– Intrinsic Heating

Most solenoid valves are designed to operate at temperatures as low as -10°C or -20°C, but the nominal limitation of 0°C is advised for any valve used in water lines. Valves to operate at -40°C (-40°F) may be ordered, where freezing is not a factor. Some models of solenoid valves are suitable for fluids with temperature up to 200°C.

The nominal ambient temperature listed are based on continuously energized conditions with maximum fluid temperature flowing in the valve. When the fluid temperature does not reach the specified maximum temperature, the actual ambient temperature in some applications may be at higher temperature than what is specified, with no harm. At continuous duty, the surface temperature of the solenoid can reach up to 120°C.

Acidity and Viscosity

Compatibility of the fluid with the valve body and internal materials must be checked, in the early stages of the solenoid valve selection. One criteria could be the pH-value of the liquid, which represents acidity or alkalinity of the aqueous solution. Pure water is neutral and has a pH value of 7. If the pH is below 7, the liquid is acidic, and above 7 identifies alkaline solutions. Strong acids have their pH below 3, and strong alkalinity starts from pH above 11.

AcidNeutralAlkaline

0-1-2-3

4-5-6

78-9-10-1112-13-14
(strong)(weak)water(weak)(strong)

The kinematic viscosity in mm2/s is a measure of the internal friction of fluids. It represents the resistance to movement of the contact surface of adjoining layers within the fluid (internal friction, viscosity of the fluid) or with different materials (external friction).

Viscosity of a fluid depends on its pressure and temperature. With constant pressure, increasing temperature decreases viscosities of liquids, while increases viscosities of gases.

Operating Voltage and Electrical Connections

solenoids are available for connection to an AC or a DC supply. Solenoids operating with alternating current (AC) are more frequently used, because of the availability of AC voltage, while the DC designs are more powerful solenoids. From a certain size, AC solenoids have disadvantages in terms of lifetime and magnetic force. 

As a standard feature, DC powered solenoids are equipped with intermediate rectifiers integrated in their socket or within the solenoids. They can be operated with a DC or an AC voltage with a frequency between 40 and 60 Hz.

The main advantage of DC solenoids is their constant current consumption, which leads to smooth switching and a coil that can cope with mechanical obstructions. The current consumption of the AC solenoids depend on the position of the core (air gap between core and pole piece). If the core is prevented from reaching its limit, the coil is overheated and can be burnt out. Should an AC solenoid designed for 50 Hz be used with 60 Hz supply, it will reduce the lifetime and the performance of the solenoid.

The voltage tolerances permitted is ±10%. Over-voltages on breaking (inductive peaks) can be avoided by connecting a varistor, diode or RC-network in parallel.

For wiring, always with power disconnected, connect the electrical cables to the solenoid in accordance with the regulations. Then close the terminal compartment to seal the cable entry properly, but not to deform the housing.  Ensure correct polarity of terminals marked + and -.  If unmarked, the power lines can be connected either way around. It is absolutely essential to connect the earth wire to the marked terminal provided.

It is advisable to carry out an operating test before pressurizing. The clicking of the plunger must be audible during switching.  Operation of the AC solenoids without the plunger causes irreparable damage.

Environmental Protection Classifications

The environment, in which the solenoid valve is to be installed, must be considered and care must be taken to order a solenoid valve with the right protection class.

The Ingress Protection (IP) code always consists of the letters IP followed by a two digit number. The first digit represents protection against penetration of solid foreign objects, while the second digit indicates resistance against liquid penetration.

1st Digit

2nd Digit
Protection Against Solid Foreign BodiesProtection Against Liquids
 0  No protection 0  No protection
 1  Objects greater than 50mm diameter 1  Vertically dripping water
 2  Objects greater than 12mm diameter 2  Angled (15°) dripping water
 3  Objects greater than 2.5mm diameter 3  Raining; maximum 60° angle
 4  Objects greater than 1.0mm diameter 4  Splashing from any direction
 5  Dust protected 5  Water jets from any direction
 6  Dust-tight 6  Heavy sea waves
 7  Immersed in water
 8  Immersed in water (specified pressure)

Special regulations have to be followed, when using solenoids in hazardous areas.

Article Source : Indumart

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