ruwl,
With respect to air failure (fail open/closed) and mechanical failure (flow opens/closes) in globe valves, when all else is equal then my opinion is that these two failures should generally be consistent if possible (i.e. specify fail close + flow closes, etc). The problem arises in many cases that globe valves do not have the desired characterisics or functionality in a flow closes configuration. In this case the flow direction is not something you are really free to specify for that type valve, and the configuration will be flow to open for that valve. The more common (and possibly plant wide) situation of air failure is then considered by itself and set to the desired failure mode resulting in the very common imstallation of: flow opens, but fail close (on loss of air pressure).
Of course every installation is a custom job, so if all types of failures must be controlled in a critical application, then alternative valve designs or shutoffs may be required.
best wishes,
sshep
If you have a process where there is tight control required, and you have to operate anywhere near the seat, flow under the seat is preferable to flow over the seat.
Flow over the seat tends to try to shut the valve, and if the actuator is air operated, the actuator has to constantly fight to balance the compressability of the air in the diaphragm against the constantly varying closing forces of the flow over the seat trying to slam the valve shut.
rmw
Thank you all for the inputs. They all are very helpful.
So, it appears that selecting a the flow-to-open or flow-to-close valve has an effect in selecting the type of actuator also?
If so, when do I specify a "piston" type actuator or "diaphragm" type actuator? Is there a general rule of thumb to specify what instrument air pressure requirement for a specific type of actuator also (3 to 15 psig or something else)?
Again, thank you all for the inputs.
Diaphragm actuators are very common to control valves. Most use air/gas in the 3-30 psi range.
Piston actuators are used more in on/off valves. They can also be used for control, but is less common. Piston actuators usually can take higher pressures, up to 80 psi. This gives them better performance for quick closing in an on/off service.
jmw said:
Don't assume the fail to open/close is the only option.
Valves can be built up with the actuator energise open/energise close (spring return) or with air to open and air to close in which case an air failure means fail in position.
I am not quire sure what you are saying - it seems you are saying the same thing in both sentences.
A fail open valve needs "something" (instrument air, instrument gas, hydraulic, etc) to close it since usually a spring holds it in the open position. A fail close valve has the spring doing the opposite.
A flow open valve has the flow direction "pushing" the valve open (ie upwards in a globe valve). A flow close valve has the flow direction "pushing" the valve close (ie downwards in a globe valve).
The only other type of fail position is fail unknown. Examples are air-to-open-and-close valves, and MOVs. Because there is no stored energy device to move the valve to one position or another, the fail position is hence unknown.
Thanks for all your replies.
I was just curious because I had seen some some old specs that have: 1) Flow to close - Failed closed 2) Flow to open - Failed Closed.
I've also noticed from the old spec that item 1) was used with angle valves where the pressure drop is large ( psig) for a letdown applications. The old spec had actually called out for a piston type actuator.
I was trying to understand the reasoning for the old spec.
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2” through 36” (50 through 900 mm)
Ratings & Connections
Flanged: ANSI 150 – , UNI-DIN 10 - 400
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Body Materials
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Actuators
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Trim
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Inherent Characteristics
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Description
The Masoneilan Series heavy duty control valve features balanced trim configurations. This product line offers efficient and stable operation under demanding process conditions, including applications with high-pressure drops and large temperature variations. Optional features include noise attenuation trim, anti-cavitation designs, and various balance seal options to meet a wide range of pressure and temperature requirements.
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