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WHAT IS IT?
This is a SERVOID® Pilot-operated, two stage, linear proportional fuel metering valve. The SERVOID® Controller is a linear proportional DC Solenoid which is used to position a button over the pilot orifice in the follower position. The Controller has a dry coil. All wetted surfaces in the unit are either stainless steel or nickel plated. There are two sliding seals on the lower follower piston; none in the Controller.
The spring system in the Controller is rated at 300 lb./inch. With hydraulic pressure amplification across it, the piston has an effective spring rate of 40,000 lb./inch resulting in an extremely stiff design, impervious to vibration with extraordinary repeatability, cycle to cycle, valve to valve.
The piston functions as a poppet which engages a fixed metal seat when the Controller is de-energized. There is no dead band between the rise and fall curves. Therefore the hysteresis over short excursion is reduced by the ratio of short to full stroke. The Controller has an external gain adjustment feature which, because of its close hydraulic coupling to the piston, provides for flow gain adjustment as well with no performance penalty. The Controller can be provided with a variety of current, coil resistance and voltage drop options. However, the power required will always be in the 4 to 6 watt range. It can also be equipped with an electrical connector in place of the leads shown. |
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HOW DOES IT WORK?
 The supply pressure is ported to the back side of the piston through a restricting orifice and to the poppet end unrestricted. When current is applied to the Controller, its resulting motion opens the pilot orifice, which is larger than the restrictor, allowing servo pressure to bleed to the pump return port. The supply pressure at the poppet end of the piston then moves toward and closes the pilot orifice causing the servo pressure to build toward supply pressure. The effective area at the back of the piston, being larger (full diameter) than at the poppet (piston diameter less seat diameter), the rising pressure causes the piston to move toward the seat. Pre-loading the Controller button against the pilot orifice insures that pressure forces always act to load the piston/poppet against the main metering orifice providing excellent leakage control, across both main and servo orifices.
The valve design is chosen when there is insufficient pressure drop across the valve itself to provide the forces necessary for efficient operation and sealing. This scheme requires a line from the Valve to the return side of the pump. In systems where there is a greater pressure drop (say 50 psid or more), the servo bleed pressure is ported to the outlet side of the valve eliminating the need for the pump return line. |
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| HOW IS IT USED? |
The basic design has superior flow resolution and turn down ratio. Typically, fluid metering valves are not accurate or repeatable in the very low flow portion of their total flow capacity. Our valves however, will deliver predictable flow from cc's to gallons per minute. This unit for instance, has a turn down ratio of approximately 1000 (max divided by min controlled flow) where 100 is usually considered good.
Some applications for which they have been used include turbine and diesel fuel controls and water metering in boiler systems. In turbine engine fuel systems, units have been supplied with flow ranges from as little as 40 to as much as 7000 pph with several in between. Units for water systems range in capacity to 16 gpm.
Given the low moving mass and high spring rate, the damped natural frequency of the system is on the order of 300 Hz. The frequency response is 10 Hz, square wave and 50% stroke. Hence, systems requiring fast response will benefit from incorporating this design.
All our metering valves can be supplied with integral delta-P regulators which will maintain a constant pressure drop across the main metering orifice. This will insure repeatable and linear flow characteristics despite changes in supply or back pressure. |
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