valve selection guidelines

7/28/2019 Valve Selection Guidelines

1/6

Valve Selection Guidelines 1

CPE10 Valve

INTRODUCTION:

When selecting a directional control valve there are 4 main criteria's to look at. The criteria's are the valve

function, the type of actuation, the installation options (mounting, wiring, and network), and the required

flow rate needed for the application

1 Valve Function:

The valve function refers to how the valves internal pathways are connected. There are four main valvefunctions for directional control valves. These valve functions are 2/2, 3/2, 5/2, and 5/3 way valves.

1.1 2/2

All 2-Way valves have a pressure supply port (1) and a working port (2). These valves come in two

different variants either Normally Closed (NC) or Normally Open (NO). When a Normally Closed valve isin its initial state, port 1 and 2 are not connected to each other until the valve is actuated. When a Normally

Open valve is in its initial state, port 1 and 2 are connected to each other until the valve is actuated. These

valves are used when it is not necessary or desirable to exhaust downstream pressure when the valve isclosed. These valves are commonly used in applications as blow off or to drive a vacuum nozzle. Most 2-

way valves have an actuator and a return spring.

1.2 3/2

All 3-Way valves have a supply port (1), working port (2), and an exhaust port (3) . They come in two

different variants either Normally Open (NO) or Normally Closed (NC). These two variants are a little

different then the same function in the 2/2 valves. When a Normally Closed valve is in its initial state, port2 and 3 are connected to each other and port 1 is blocked until the valve is actuated. When the valve is

actuated port 1 is connected to the port 2 and the port 3 is blocked. When a Normally Open valve is in its

initial state, port 1 and 2 are connected to each other and port 3 is blocked until the valve is actuated. When

the valve is actuated port 3 is connected to port 2 and port 3 is blocked. These valves are most commonlyused when it is necessary to exhaust downstream pressure through the valve. For this purpose, port 3 is

usually returned to atmosphere through a pipe port, or an orifice (vent) in the body of the valve. Forexample, this valve can be used to control single-acting actuators (cylinders, rotary actuators, grippers,

etc,,), which must be exhausted to atmosphere in order to return to their un-actuated position. These valveswill have an actuator and a mechanical spring or air spring to return the valve to its initial position. If Port 3

is plugged, the 3/2 valve will function identically to a 2/2 valve.

1.3 5/2

All 5/2 Way valves have a supply port (1), 2 working ports (2,4), and 2 exhaust ports (3,5). These valves

are most commonly used to control the position of double-acting actuators. For this purpose, the working

ports (2, 4) are connected to the actuator, and the exhaust ports (3, 5) are usually returned to atmospherethrough pipe ports, or orifices (vents) in the body of the valve. A 5/2 valve always has one of the working

ports (2,4) connected to the pressure supply port (1), and the other working port is always connected to an


2/6


exhaust port (3,5) which is connected to atmosphere, regardless of the position of the valve. 5/2 valves

come in two different configurations single actuated or double actuated valves. Single actuated valves willeither have a mechanical spring or an air spring to return the valve to its initial position. A double actuated

valve uses two actuators to control the position of the valve. Often double actuated valves are referred to as"Memory" valves because the valve will not return to its initial position until one of the coils are actuated.

1.4 5/3

Like 5/2 way valves 5/3 valves have a supply port (1), 2 working ports (2,4), and 2 exhaust ports. The main

difference is that the 5/3 has 3 possible positons.5/3 valves have an additional spring-centered neutral

position, which the valve takes when both actuators are not engaged.. In the neutral position, both working

ports (2,4) may be blocked (5/3G), pressurized (5/3B), or exhausted (5/3E) depending on the specific type

of 5/3 valve. In the blocked version, none of the ports are connected to each other. In the pressurizedversion, the supply port is connected to both the working ports (2,4) and the exhaust ports are blocked. In

the exhausted version, the working ports(2,4) are connected to the exhaust ports(3,5) and the supply port 1

is blocked.

2 ACTUATOR TYPES:

2.1 Solenoid Actuators

2.1.1 Direct-Acting

Direct acting solenoid valves are shifted exclusively by the direct mechanical action of a solenoid coil .This means that either the plunger in the coil is used in the valve as poppet or the solenoid plunger is

connected to the poppet or spool in the main part of the valve.. These types of valves usually have

substantially larger solenoid coils than comparable valves of the Pilot Assist design (see below). The size

of the solenoid grows dramatically with increased pressure and flow capacity ratings. This often results in

large valves that generate substantial amounts of heat. As a result, some direct-acting solenoid valves are

not rated for continuous duty. Direct-acting solenoid valves are used in applications where low-flow

switching at high speeds is required. However, while it is true that direct-acting valves shift much fasterthan Pilot-Assist valves, direct-acting valves have much less flow capacity for an equivalently-sized

solenoid. What matters in the end is how quickly can the necessary volume of air, at a given pressure, bedelivered to the component or system downstream. For this reason, a slower pilot-assist valve (with a

higher flow-rate) can move an actuator much more quickly than a fast direct-acting valve (with a lower

flow rate) can.

2.1.2 Pilot-Assist

Unlike Direct-Acting solenoid valves, Pilot Assist valves have no mechanical linkage between the solenoid

plunger and the main spool of the valve. The shifting of the main spool is, instead, accomplished by air

pressure acting on an integral piston or diaphragm. This arrangement can therefore be regarded as twovalves in one. A very small Direct-Acting 3/2 solenoid valve controls the pilot air going to a much larger,

air-actuated main valve. This is done, even in small valves, to obtain significant reduction in the

solenoids size, energy consumption and heat generation. Since the air volume required to shift even alarge air-piloted valve is small, a small solenoid-actuated pilot valve will suffice. Most solenoid-operatedvalves used in todays industrial applications are of the Pilot-Assist design. These units come in two basic

configurations; Internal and External Pilot supplies.

2.1.2.1 Internally Piloted

The solenoid actuator of an internal pilot supply valve gets its air from the main working supplyport (1). There is an internal connection that diverts a small amount of air from Port 1 up to thesolenoid head. This means that there must be a minimum pressure (control pressure) present onPort 1 at all times in order for the valve to work properly. This requirement prevents these valvesfrom being used with low pressures and vacuum, or being used in unconventional supply


3/6


pressure arrangement, such as using the working ports as the supply ports and using the supplyport as a working port. This is sometimes done so that two different pressures can be selected

2.1.2.2 Externally PilotedExternal pilot supply valves have an extra connection port(s) for supplying air exclusively to the solenoid

actuator (and return air-spring, where applicable). In this case, the supply port (1) is no longer required tohave a minimum pressure present. This allows valves of this type to be used for controlling vacuum and

low pressure circuits, creating separate pressure zones, and a host of other creative or unconventional

applications. Regardless of application, however, there must always be present available supplies of pilot

air at no less than the minimum specified (control) pressure.

2.2 Mechanical/Manual Actuation

Another method of valve actuation in which some sort a of lever or push button, or knob is used to change

the position of the valve. Mechanically/Manually actuated valves are often used when a machine requires

human intervention to run a machine or when an pneumatic output is needed to start another process in a

machine.

2.3 Air Piloted Actuation

Air actuated valves are valves which require a pilot pressure so that the valve may be switched. These

valves always have external pilot ports on them. They are often used in applications where solenoid valvesare not permitted due to safety reasons. These valves get pilot pressure from external valves. They can

either be actuated via a manual valve or a solenoid valve, which is connected to its pilot ports.

3 Installation Options:

How valves are to be installed in the system often drives the selection process. Valves may or may not beeasily accessible, the environment may be hostile, or there may be special requirements for maintenance.

There are 2 main installation options for valves stand-alone valves and manifold mounted valves.

3.1 Stand Alone

Stand-alone valves can be categorized in 3 different ways In-Line, Semi In-Line, and subbase mounted. In-

line refers to a valves where all of the ports are part of the body of the valve. The working ports are usual

on the one side of the valve and the exhaust ports and supply port are usually on the side parallel to theworking ports. Semi-inline valve refers to valves, which are mounted on a subbase and have the working

ports coming out the top of the valve and the other ports being part of the subbase. A subbase valve is a

valve where all of the ports are part of the subbase and are not located on the valve external walls. In-linevalves are often used for individual installations where the actuator performance can be improved by

locating it physically close to the valve. In applications where there are a few widely scattered actuators,

in-line installations are usually preferred over manifolds. Sub-base mounted offer the additional benefit ofsimplified maintenance and system repair, since the system plumbing to the sub-base need not be disturbed

when replacing individual valves. Sub-base valves are especially attractive when rigid system piping is

used.

3.2 Manifold

Manifold mounted valves are banks of valves that are mounted together and share exhaust channels and

supply channels. There are two different types of manifolds, extruded and subbase manifolds. Extruded

manifolds are manifolds created from long extrusions where holes are drilled in the manifold for attaching

several valves. Subbase manifolds are manifolds which consist of several subbase connected together to

create a manifold. Manifolds also come in two sub-divisions Semi-Inline and with the working ports as partof the manifold. One of the more interesting developments in solenoid valve manifold design is the

integration of electrical and electronic devices into the manifold assembly. For example, valve manifolds

that have plug-in solenoids can have all electrical connections going to a single multi-pin connector, thussimplifying installation. Other integration techniques include the incorporation of various common

industrial PLC network transceivers. This technique allows large numbers of solenoids to connect to a


4/6


remote PLC using a small two-wire high-speed data cable, thereby drastically reducing the amount of

installed system wiring that would otherwise be required. These so-called Fieldbus manifolds can evenbe fitted with powered electrical receptacles for connecting proximity sensors and low-powered electrical

output devices.

4 FLOW CAPACITY

Once the type of valve is selected, the next most important selection criterion is the valves flow capacity.

Flow, which is measured as Volume Coefficient (Cv) or in Liters per Minute of flow, is an indication of the

amount of resistance a valve presents to a pneumatic circuit. All devices that conduct air will resist flow to

some degree, which causes the pressure drop across the device to increase as the flow increases. The lessresistance, the smaller the pressure drop.

It must be noted that any device, fitting, or run of tubing can effect the flow rate in a system. In highlytime-critical applications, a few extra inches of tubing, or the wrong fitting can mean the difference

between a circuit that works and one that does not. For this reason, valve flow ratings alone cannot predict

the flow rate through a system branch. In the past, the common practice used in selecting a valve was to

match the port size of an actuator and the valve. This is no longer a suggested way of sizing a valve to an

actuator. Advancement in technology has allowed smaller valves to have increased flow. Smaller valvestend to have several advantages over larger valves. In general, the advantages of smaller valves are quickerswitching time, lower leakage, less expensive and generally lower power consumption since smaller

solenoids are generally used. With these advantages, smaller valves will initially save money and then save

money due to decreased leakage in a system and decreased power use.

In order to select a valve the flow required to move the actuator in the time required by the

application. This flow required can be found by using the following equation.

US UNITS

Q= (Volume x Compression Factor)/(Time x 28.8)

SI UNITS

Q=(Volume x Compression Factor)/Time

Q Flow Rate in ft3/min or liter/sec

Volume in3

Time sec

Compression Factor (P1 + Pa)/Pa

P1 Inlet Pressure in psi or bar

Pa Atmospheric Pressure 14.7psi or 1bar

After the flow is calculated the Cv required can be calculated using the following equations.

US UNITS

CvQ

22.48

T G.( )

P P 2 P a.

.

SI UNITS


5/6


CvQ

114.5

T G.( )

P P 2 P a.

.

T Temperature in Rankine or Kelvin

G Specific Gravity (G=1 for air)

P Pressure drop across the valve in psi or bar

P2 Outlet Pressure in psi or bar

Pa Atmosphere Pressure 14.7psi or 1bar

EXAMPLE:

A double acting cylinder with a 25mm bore and 100mm stroke

is extended with a pressure of 6 bar. The rod of the cylinder

has a 10mm diameter. The cylinder is required to extend in

0.25 seconds and return in 0.2 seconds. What Cv is requiredto extend cylinder and retract the cylinder?

Step 1 : Calculate the areas and volumes

A extend 25

2

2.

A extend 490.874=

A retract

25

2

2.

10

2

2.

A retract 412.334=

V extend A extend 100.

V extend 4.909 10

4.

=

V retract A retract 100.

V retract 4.123 10

4.

=

Step 2 : Calculate the Compression Factor

CF6 1

1 CF 7=

Step 3 : Calculate the flow rate

Q extend

V extend CF.

0.25( ) Q extend 1.374

Q retract

V retract CF.

0.2( ) Q retract 1.443

Step 4 : Calculate the Cv

Cv extend

Q extend

114.5

293 1.( )

0.25 5.75 1( ).(

.

Cv extend 0.158=


6/6

valve selection guidelines

Documents