Non Bleed Positioners 1

[tsenthil]

What is Non Bleeding Type Positioners? Can anybody explain in details.

I thought all positioners has to bleed while in operation.

 

[JimCasey]

Spool-valve positioners bleed because there is a small clearance between the spool and the block. The bleed air actually serves as an air bearing. It's not altogether a waste because the bleed air purges the housing and the positioner can live in a nasty environment. Look at all the PMV positioners in Paper mills.

Poppet valve positioners leak a smaller amount of air because if they are assembled carefully the poppets can both be closed at an equilibrium position. "Bleed" is not designed in, but small leakage at equilibrium occurs. Poppet valve positioners include the Masoneilan 7700 used on top of the Domotor actuators, and the Valtek XL extreme precision analog positioner.

Many Digital positioners use piezo valves and an amplifying block. The piezos and the block can shut off bubble-tight and thus there is NO bleed. Some digital positioners juat have a digital control box on an old-style force-balance spool-valve positioner. "Digital" does not automatically mean "Zero Bleed". Digital Piezo/Zero Bleed positioners include the Siemens Sipart, PMV D3, and the Flowserve Logix 500.

 

[Compositepro]

The bleed that I am familiar with seeing is from a pneumatic amplifier in the positioner. Air is bled through a orifice and then a tiny nozzle to atmosphere. An electrical or pneumatic signal will change the position of a flapper valve over the nozzle which changes the back-pressure between the orifice and nozzle. This pressure will vary between supply pressure and atmospheric. The pressure is used to operate an actuator which moves the nozzle to a new position in equilibrium with the control signal.

Look-up pneumatic amplifiers for more details.

 

[tsenthil]

Jim,

Its sounds very basic but please explain

1. Is Spool Valve and Flapper/Nozzle design(Fisher DVC) same?

2. Piezo valve is the same technique used in Solenoid Valves?

3. Some vendors (Siemens Sipart)claims their positioner as Fail Freeze. Is this related to Zero Bleed?

 

[JimCasey]

Spool valve is a sliding cylindrical member inside a block, THere are ports on either side of neutral position. If the spool is deflected one way, it admits air to the valve actuator. If the spool is deflected to the other side of neutral, it vents air FROM the actuator. In the neutral positon it just ....bleeds.

Flapper nozzle is variable back pressure. WHen the flapper closes down on top of the nozzle the backpressure (may be connected to an output) increases. If the flapper lifts and opens, the pressure decreases. It's possible to get 3-15 psi this way, but I have never seen a flapper nozzle that will give the 0-80 psi kind of output that modern positioners have. But the designers could put a pneumatic relay under the flapper-nozzle to get any kind of output desired. Masoneilan used to use flapper-nozzles in their 2700 series pressure controller and their 8006/8012 transducer/positioner. Vibration may cause the flapper to impact on the nozzle, causing fretting and calibration shift. Masoneilan introduced a modification to avoid just that problem in 1976. ( DO I sound OLD now?)

A piezo valve uses a semiconductor piezoelectric beam that changes shape as a voltage is applied. The current draw is very tiny thus it can be energized by a 4-20 ma signal. Conversely the capacity of a piezo valve is very tiny but, like a flapper-nozzle, it sits atop a pneumatic relay/amplifier to pass the sort of flow and pressure needed. You mentioned solenoid valves: A solenoid uses a relatively greater amount of energy flowing thru a coil to change the valve position magnetically. The only valve positioner I know of with solenoid valves is the Leslie DVC. Because of the energy requirement of the solenoids it must be a 4-wire device, not loop powered.

Fail freeze is not exactly related to zero-bleed, but one is necessary for the other. The fail-freeze option causes the piezo valves to lock closed if the signal goes below 4 ma. (user-assignable value, but 4 ma is common). NORMAL operation of a positioner is for the positioner to interpret anything less than 4 ma as ZERO and to vent the actuator. SO if you give a failfreeze positioner 4 ma the valve closes. But if you are running along midscale and the wire gets cut, the valve stays in last position. Siemens is not the only failfreeze positoner. PMV has it on the D3 as an option. There may be others.