What is the main functional difference between a pneumatic diaphragm actuator & a pneumatic piston actuator while operatin a globe valve.Will it be difficult to attain the precise control with piston actuator?Safety valves

For on-off valve (isolating valve) : no.

For a regulating valve the answer is somwhat more complicated, depending on the position-feedback system from the valve, the total construction diphragm to globe and piston to globe.

Plug ValvesFor a diaphragm operated regulating valve the diaphragm could be larger (in area) than the piston in piston operated valve, and the globe more or less directly attached to the diphragm. Both indicating a more direct connection between pneumatic pressure and position.

For a piston operated valve with seal friction for stem and piston seals you would have to depend on a mechanical position feedback. These will cost a lot, give an additional and often unprecise element (depending om price, make and construction) and as a rule need a constant amount of air to function.

This was true once but is seldom true these days.  Or the spring is set for a bench range of something much biased from 3-15 to compensate for process effects on the valve when it is pressurized. The output of a positioner is NOT 3-15; it is zero to supply pressure and the positioer applies any pressure necessary to make the valve move to its desired position. A straight 3-15 control signal is unlikely to move the valve to its needed position.  3-15 positioners only make up single-digit percent of the worldwide mix, with 4-20 ma far greater and digital growing exponentially.  You can't turn a bypass valve and make the pneumatic actuator run on a 4-20 signal.

Reducing ValvesDiaphragm actuators evolved when valves were operated directly with the air signal from a controller. Nominally the signal range is 3-15 psi.  

It is then necessary for the actuators to have large diameter to genrerate enough force to move the valve.  
Diaphragm actuators are sensitive when considered in an open loop.i.e.: you will get a small motion from one with a small change in signal.  However in a closed loop (attached to a valve) they are not as ideal.  Packing friction in the valve is then the limiting factor for sensitivity.  A large-diameter low pressure actuator lacks stiffness (Think "waterbed") so a small incremental change in air pressure is less likely to overcome packing friction and conversely process pulsations are likely to be able to compress the low-pressue air in the diaphragm chamber so the valve can be moved by the process.  
Positioners help, but the air pressure is still limited typically to 60 psi (4 bar) and the stamped metal case, diaphragm plate, and rubber diaphragm all deflect with changes in pressure.  

Compare to a cylinder: The cylinder uses a positioner and much higher pressure air-typically 80 psi(5.5 bar) , up to 120 (~8.5 bar) .  High-pressure air is stiffer.  Cylinder actuators have smaller diameter because the higher-pressure air generates as much force against a smaller area. The friction of the piston seal in the cylinder actuator is still no greater than the packing friction of the valve. Balance ValvesIf you send the positioner a small incremental change in signal it meters in air to the cylinder at up to full supply pressure until the valve moves to a new position corresponding to the new signal.  With the smaller volume of a cylinder, if the process pulsations push back on the valve plug, the change in air pressure (force) per amount of deflection is greater than a diaphragm, particularly (exponetially) at the critical positions near the ends of travel.
The piston is very rigid compared to the diaphragm plate, diaphragm housing, and relatively stretchy diaphragm.   
With less volume in a cylinder it is able to respond more quickly.

Diaphragm actuators evolved when valves were operated directly with the air signal from a controller. Nominally the signal range is 3-15 psi.  

It is then necessary for the actuators to have large diameter to genrerate enough force to move the valve.  
Diaphragm actuators are sensitive when considered in an open loop.i.e.: you will get a small motion from one with a small change in signal.  However in a closed loop (attached to a valve) they are not as ideal.  Packing friction in the valve is then the limiting factor for sensitivity.  A large-diameter low pressure actuator lacks stiffness (Think "waterbed") so a small incremental change in air pressure is less likely to overcome packing friction and conversely process pulsations are likely to be able to compress the low-pressue air in the diaphragm chamber so the valve can be moved by the process.  
Positioners help, but the air pressure is still limited typically to 60 psi (4 bar) and the stamped metal case, diaphragm plate, and rubber diaphragm all deflect with changes in pressure.  

Compare to a cylinder: The cylinder uses a positioner and much higher pressure air-typically 80 psi(5.5 bar) , up to 120 (~8.5 bar) .  High-pressure air is stiffer.  Cylinder actuators have smaller diameter because the higher-pressure air generates as much force against a smaller area. The friction of the piston seal in the cylinder actuator is still no greater than the packing friction of the valve. If you send the positioner a small incremental change in signal it meters in air to the cylinder at up to full supply pressure until the valve moves to a new position corresponding to the new signal.  With the smaller volume of a cylinder, if the process pulsations push back on the valve plug, the change in air pressure (force) per amount of deflection is greater than a diaphragm, particularly (exponetially) at the critical positions near the ends of travel.
The piston is very rigid compared to the diaphragm plate, diaphragm housing, and relatively stretchy diaphragm.   
With less volume in a cylinder it is able to respond more quickly. API Cast Steel Valves

Important advantage of Spring Diaphragm Actuator is that for most of the actuator the spring range will be 3-15 psi and in case of positioner failure, the signal pressure can be directly fed to Actuator for control.

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