Vm Wntr06 Solenoid Pilot Valves Lr

8/7/2019 Vm Wntr06 Solenoid Pilot Valves Lr

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W I N T ER 2 0 0 6

V O L U M E 1 8 , N O . 1

The LNG

ChallengePLUS:

Extreme Valves

The Butterfly Valve Evolves

Do We Need Valve

Qualification Programs?

The Ins and Outs of

Solenoid Pilot Valves

OEM Vs. Non-OEM Parts

The LNG

ChallengePLUS:

Extreme Valves

The Butterfly Valve Evolves

Do We Need Valve

Qualification Programs?

The Ins and Outs of

Solenoid Pilot Valves

OEM Vs. Non-OEM Parts



C2 | Valve M A G A Z I N E

The term “pilot valve” is used inreference to a three- or four-way

solenoid valve that pilots or controlsthe flow of media, such as air orwater, into an actuator, which in turncontrols the movement of a valve con-nected to the actuator. In otherwords, the pilot valve drives or con-trols the main process or control valvein a process control system.

To meet specific customer needs,pilot valves are available in a multi-tude of variations. Electrically, theyvary by voltage, power consump-tion, and method of protection.Most solenoids can be designed foruse in almost any voltage, whetherit’s AC or DC. The trend in the mar-ket is toward DC because ofincreased use of distributed controlsystems and bus protocols. The onlydrawback in using DC is that mostsolenoid valves in DC valves cannotoperate at the same pressure astheir AC equivalents. However,

some manufacturers now have tech-nology that permits the same pres-sure rating whether the valve is ACor DC. In fact, solenoid coil tech-nology has improved so much that acustomer can purchase one valveand operate it over a given voltagerange such as 100 to 240 at 50 to60Hz in AC or DC.

SOLENOID PILOT VALVES FOR

VALVE ACTUATIONKNOWINGTHE INS AND OUTS OF PILOT

VALVESWILLHELPUSERSSELECTTHE

CORRECT PILOTVALVECONSTRUCTIONFOR

ANAPPLICATION.

BY BILL REESON

Design ChallengesOver the past decade, one of the biggest challenges of solenoid design has been min-imizing power consumption. This is especially true for pilot valves. The increaseduse of distributed controls systems and bus protocols has meant the average sole-noid power consumption in DC voltage has dropped from a level of 10 watts to 1.5watts. The less power consumed, the smaller and less expensive a power supplyneed be. Bus protocols are demanding more stringent power levels than ever. Theless power consumed over a bus network, the more nodes or slave devices are possi-ble over that network. If the protocol is powering the solenoid directly off the bus,power restrictions are limited from 0.5 watts to as low as 1.4 milliamps, dependingon the type of network. As soon as the power consumption drops tothe milliamp range, the technology shifts from a typicalelectromagnetic circuit solenoid to a piezo orconstant bleed device. Powering the pilot valvedirectly off the bus via a position indicator givesthe user less wiring since only one cable is con-

nected from node to node, eliminating the needto run a separate cable to each device from aPLC. This one cable transmits data and power.To use a pilot valve that cannot be powered off ofthe bus would require an external power supplyfrom the bus and in many cases a different nodedevice that will allow use of an external power sup-ply. Pilot valves that can be used as individual

nodes or slave devices over certain net-work protocols are also available. How-

ever, they are not as common because mostpilot valves used over networks are standard pilot

valves that serve as an output device from a posi-tion indicator.

Depending on the environment, many methods ofprotection are used. A large part of the process controlmarket requires explosion-proof equipment. To meetthat requirement, the pilot can be designed using intrinsic safety in such a mannerthat the solenoid cannot cause a spark or generate sufficient heat to create an ignitionsource. Another protection method is to contain possible sparks or explosions withinan enclosure that will not allow flame to propagate to the outside atmosphere. Theseproducts either have the explosion-proof enclosure or use electronics that are encapsu-lated with some type of potting compound. Third-party approval for these protectionscan be achieved, depending on where the customer is located.

Valve with a voltage ranging coil

Piezo-operated pilot valve for

use as a Foundation fieldbus

or Profibus PA output device



W i n t e r 2 0 0 6 | C3

Pilot Valve FunctionsPilot valve function has typically beeneither three- or four-way. Today thereare many functions based on these two

basic descriptions, and it is not uncom-mon to see descriptions such as 3/2NC, 3/2 NO, 3/2 U, 3/3, 4/2, 5/2, and5/3. While these designations may lookconfusing, they are actually quite sim-ple. For a three-way pilot valve, forexample, the expression 3/2 NC means:

The number 3 signifies threeports on the valve.

The number 2 designates that thevalve has two positions. The twopositions are energized or de-energized.

NC stands for normally closed,which means that in the de-ener-gized position, the process con-nection to the actuator is closed.

The first port is normally marked asP (pressure) or 1. This is the air supplyto the valve. The second port is C(cylinder) or 2. This is the working portconnected to the piston side of aspring-return actuator. The third port isdesignated 3 or E (exhaust). The

exhaust port is where the air from theactuator is released when the pilotvalve changes state from energized tode-energized.

Sometimes there is another port ona 3/2 pilot valve. This most likely willbe designated as R, which stands forre-breather—the port connected to thespring side of a spring-return actuator.As the air being exhausted travels outof the piston side of the actuator, there-breather port channels it into the

spring side. This keeps outside air frombeing ingested into the spring side ofthe actuator, giving that actuator someenvironmental protection from any-thing in the atmosphere that may cor-rode internal parts.

Four-way pilot valves typically areused for double-acting actuators. Apilot valve designated 4/2 is a four-wayvalve with four ports. It consists of anair supply, designated 1 or P; two cylin-der or working ports, designated ports

2 and 4, or A and B; and a com-mon exhaust port for both of theworking ports, designated 3 or E.

A 5/2 valve is a four-way valve

that has five ports designated as fol-lows: Port 1 is the air inlet or pressureport, ports 2 and 4 are the cylinder orworking ports, and ports 3 and 5 arethe exhausts. Port 3 is the exhaust forport 2 and port 5 is the exhaust forport 4. Each cylinder port has its ownexhaust so you can add flow controllersto the exhaust ports to control theopening or closing speed of the actua-tor that the pilot valve controls.

Single or Dual SolenoidsAll the constructions described aboveare available with a single solenoid ordual solenoids. The single-solenoid ver-sions normally have a spring return sothe valve automatically returns to itsnormal state during power loss. Thedual-solenoid versions remain in thelast position when power is lost.

With the dual-solenoid versions,there are constructions designated as3/3 and 5/3. Again, the second numberdesignates number of positions. The

positions are the following: de-ener-gized or center position; solenoid Aenergized; and solenoid B energized. Inthis dual-solenoid construction, thecenter position is the only position thatis different. Typically, this center posi-tion is called “center open,” where allof the cylinder ports to exhaust areopen, or “center closed,” where all ofthe ports are closed. The center posi-tion can be made to accommodatealmost any function—depending on

customer needs—that the pilot valvedesign will allow. The majority of pilotvalves for this function are spool valvesused for keeping the actuator frommoving from its position during powerloss.

Mounting ConfigurationsPilot valves also are available in manymounting configurations. The mostcommon are piped, NAMUR-mounted,pilot valve islands, and pilot valves

integrated into a position indicator orintegrated as part of the actuator itself.The overwhelming majority of pilotvalves sold each year are piped. How-ever, the other configurations are rap-idly growing in use.

Perhaps the fastest growing configu-ration is the NAMUR standard mount-ing, which started in Europe more thantwo decades ago. This standard allowsthe customer to save on piping costsbecause the pilot valve can be attachedto the actuator without the need forpiping. The only pipe needed is for airsupply to the valve. As actuator manu-facturers accepted this standard, so toohave pilot valve companies. The twosizes for the NAMUR mounting for

pilot valves are 1/4-inch and 1/2-inchstandards. The 1/2-inch is not as com-mon right now, but actuators and pilotvalves in this size are available, as arethe more common 1/4-inch NAMURstandard for control valve applications.The difference between this standardand the normal 1/4-inch standard is aport included for external pilot air.

Pilot Valve IslandsThe pharmaceutical industry uses pilot

valve islands (or manifolds), where theprimary function is the actuation ofsanitary valves. Since the ingress pro-tection and approvals of these types ofvalves are limited, pilot valve islandsare mounted inside an enclosure. Allthe electronics reside in the enclosure,and tubing runs from the enclosure toeach point of actuation. A benefit ofthese islands is that the pilot valvesusually are in one location, with onepoint of electrical connection as well as

NAMUR mount valve



one point ofpneumatic con-nection for the supply.Pilot valves built into or attached to aposition indicator are very commontoday. Typically, the pilot valve is con-nected to a position indicator via con-duit, so it makes sense to integratethese two pieces of hardware into one.This approach gives the customer a sav-ings in electrical hookup costs and gen-erally saves space on the overallassembly.

Pilot valves integrated into actuators

also are available. These combine thefunctions of an actuator, position indi-cator, and solenoid pilot valve in oneunit, an approach used by some actua-tor manufacturers to supply one turnkeysolution for the entire assembly. All thatis needed is one electrical connectionand one pneumatic connection.

Basic Construction:Poppet or SpoolThe basic construction of a pilot valve

is either poppet or spool, which refersto the actual internal workings of apilot valve that diverts air from thepressure port to the cylinder and thecylinder to exhaust. In the past, mostpilot valves were poppet construction.Poppet-style pilot valves consist of anelastomeric disc that opens and closesand an orifice—a very simple and reli-able approach to controlling the flowof a fluid.

The individual pilot valves found in

valve island applications are of spool

design, which is popularbecause it is generally

less expensive to con-struct. One spool slidesthrough a series of O-

rings. Groovesmachined into thespool act as flow passages,

transferring the flow of the air fromone port to another. When spoolvalves were first used in the process

control market, some customers hadconcerns about the relationship of fric-tion to the reliability of these pilotvalves. Some spool valves did not shiftbecause of the inherent friction of thedesign. As a result, some pilot valvemanufacturers have conducted exten-sive product development to overcomethis.

Generally, customers prefer onevalve construction over another. Manylike the poppet-style valve becausemovement is in direct relationship tothe movement of the solenoid armature.These direct-acting pilot valves do notrequire minimum air pressure to oper-ate. For this reason, this style of valveis used to operate dome-actuated con-trol valves that operate between 0-15

psi. Spool valves are considered indi-rect-acting valves, since they require aminimum amount of air pressure tocause the spool to move. Actually, mostspool-type pilot valves are operated by asmall, three-way, direct-acting valvethat transfers air pressure to a piston,which then pushes the spool and causesmovement. Indirect-acting valves canbe used on control valve applications aswell, but the valve must have an exter-nal pilot connection. Air pressure suffi-

cient to shift the valve is connected tothe external pilot connection, while themain valve body controls the flow oflow pressure air into the control valve.

Safety and ReliabilityConcernsSafety and reliability also have been thefocus of industry concern. Over theyears, pilot valves used in many appli-cations were for systems designed by anassembler or end user. The end userwould pipe up two solenoid valvestogether so that if one of them were tode-energize unexpectedly, the othervalve would keep the process from shut-

ting down prematurely. This may be areliable solution, but it is not necessar-ily safe. If the system is asked to shutdown, the risk doubles because twosolenoids are closing. Today, users canpurchase a complete pilot valve systemthat will operate with the redundancyneeded to keep the process running, butthat will also shut down upon request.Such systems can test the solenoidvalves as well as partial stroke to testthe process valve for system integrity.

To correctly fit a customer’s applica-tion, many aspects of pilot valves mustbe considered. Most solenoid valvemanufacturers have customer supportpersonnel and distribution networks tohelp users select the correct pilot valveconstruction for their applications. VM

BILL REESOn is responsible for process control mar-

keting and business development with Emerson

Industrial Automation,ASCOValve. Reach him at

[email protected].

C4 | Valve M A G A Z I N E

S O L E N O I D P I L O T V A L V E S F O R V A L V E A C T U A T I O N

Pilot valve island

Actuator with built-in position indicator

and solenoid valve

Redundant solenoid system for reliability

and safety

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Vm Wntr06 Solenoid Pilot Valves Lr