Obtaining reliable shutoff valving of liquid flows
November 14, 2003
Piston Actuated Valves (PAVs) are a good choice wherever quick-responding, open/close operation with positive sealing is necessary for precise flow control. As such, they are an attractive alternative to both solenoid and ball valves, particularly under special conditions:
• For precise batch control
• For precision filling operations
• For aggressive or troublesome liquid flows
• For reliable long-life operation
In particular, Piston Actuated Valves offer reliable flow control with light slurries, viscous fluids, corrosives, saturated steam, hot oils, adhesives, thixotropic fluids, carbonate fillers and sweeteners.
PAVs are two-way valves that are offered in normally closed, normally open and bi-directional operation. The bi-directional variant is recommended exclusively for high-pressure water applications to eliminate waterhammer. Positive sealing is aided by a soft Teflon® main seal.
PAVs function well under a broad operating pressure range of 0–290 psi, temperatures to more than 350 degrees F and vacuum to 29'''' Hg. Their angled seat design affords the flowing media a smooth transition through the valve, resulting in high flow rates with minimum pressure drops. They attain maximum flow quickly under conservative pumping load for attractive energy savings in rapidly cycling applications.
PAVs are robust in their materials and construction with an estimated service life of up to 5 million-plus cycles. Body materials include 316L stainless steel or bronze with a brass bonnet. Wetted materials are Teflon, Niploy coated stainless steel with 316L stainless steel or bronze and brass. Their design is compact, enabling the valves to be installed in tight spaces.
In slurry applications, PAVs offer longer life than ball valves, a design characterized by close tolerances and sliding contact. Scoring and erosion caused by abrasive slurries can degrade the ability of these types to maintain a lasting seal, making consistent flow control difficult. The poppet valve design of the PAV has no sliding contact, making erosion its only wear mode. The smooth flow transition through the valve minimizes erosion effects. Furthermore, the sealing components are easily replaceable without special tools or training.
Since gas or fluid pressure provides the actuating force, there are no electric components proximal to a potentially explosive environment. This gives the PAV a distinct advantage over solenoid valves in applications involving volatile hydrocarbons.
PAVs can be easily sequenced in delivery/shut-off and bypass loop combinations that protect especially shear-sensitive products from degradation by fixed-displacement pumps. With this set-up, PAVs used in packaging applications can provide consistent filling of viscous products by mass or volume. They can also be sequenced in batching operations to provide extremely accurate dosing.
In lines where waterhammer is a problem, the bi-directional version with air actuation serves as an air spring to absorb the energy transmitted by the wave front, protecting both the valve itself and upstream components.
Valve stems with self-aligning glands and a floating seal combine for high sealing integrity in applications where contamination cannot be tolerated. The seal and gland design also makes the PAV well suited for use in reactors where heating and cooling media must cycle through the same lines sequentially. The PAV is highly tolerant of heating and cooling cycles. Its stem housing or bonnet is highly effective in dissipating heat.
This class of valves is available in line sizes from 1/2'''' to 2'''' NPT and BSP. Other connection options include butt- and socket-weld along with flange and Tri-Clamp. Three actuator diameters are available to accommodate various flow rates and pressures.
Planning the application
The first step in applying PAVs is to establish the line media pressure and media temperature. Then, determine the pilot pressure and its source. Assuming the application falls within the specifications of the PAV, the next step is to establish how the pilot pressure is to be delivered to the PAV through pilot valving, which may be local (assembled directly to the PAV actuator) or remote (from a panel or distant location). With the pilot valve in a remote location, the PAV is completely safe in explosive environments.
If the valves are to be sequenced, the pilot valves must be interfaced with digital controllers. A good starting point is to sketch out a logic diagram incorporating all inputs and outputs from the controller to the valves, pumps and any other system components. The state of each valve (open or closed) during every stage of the process should likewise be established at this time. Also, any transducers required should be identified, specified and positioned within the system design. Automation consultants can assist with this stage of the project as necessary.
The local pilot valve is specifically designed and sized to match each of the three actuator diameters. Remote pilot valves can be a single valve application or configured in a banked valve series for central control of multiple PAVs. A useful control option, the travel switch indicates the state of the valve open or closed. PAVs can incorporate a stroke regulator, which controls the amount of media flow through the valve as well as acting as a manual override.
A good choice
Where positive shut-off and tight sealing of a difficult media flow is critical to accurate batching, dosing or package filling, PAVs are a good choice. They offer reliable performance in environments hostile to electrical components or electronics, with easily established control and long service life.
For more information, contact Spirax Sarco, Inc., at 803-714-2000.