MAST value 1

[sini4s]

What is MAST value & how does it affect the actuator selection for ball & butterfly valves?
As per my understanding:
MAST: MAST(Maximum allowable stem shear torque).
hence while going for the actuator selection,one can summarized as below:-
Ball valves/ Butterfly valves:
Fail to close
BTO < AST
ETC< SET < MAST
Running torque< ART & SRT

Fail to open
BTO < SST
ETC< AET< MAST
Running torque< ART & SRT

The MAST comes into play while fail safe close/open action hence it should be as above.

 

[bcd]

Hello,

MAST is maximum allowable stem torque. It has nothing to do with the valve oeprating torque. It is the maximum torque that can be applied to the stem without permanently deforming it. In a perfect world, the maximum actuator output torque would be less than the MAST so that the actuator can never twist the stem and prevent proper seating of the valve.

 

[sini4s]

Hello Bcd,

Thanks for the reply. yes the MAST has to do with the actuator torque but my question is which of the torque should be considered AST, AET, SST & SET.
Also as seen my post, fail safe position plays a role.
if AST > MAST for fail safe close, then the actuator will work or not.

 

[bcd]

There are two items to consider when selecting the actuator size. First, the actuator must provide enough torque to fully cycle the valve. Secondly, the maximum possible actuator output torque must be less than the MAST. Look at each condition separately. First with selecting the actuator for the valve opearting torque. Then ensure the stem size and material strength provide a sufficient MAST.

 

[sini4s]

I am really sorry for not making myself clear over the doubt.
For example
one ball valve with FC position
Torque
BTO 36000N.m
ETC 14400N.m
ETO 14400N.m

AST: 60000 N.m
AET: 18000 N.m
SST: 36000 N.m
SET:24000 n.m

if MAST value is 58584 N.m will the actuator work.
above is hypothetical data.

 

[PeterIgg]

Ok, to clarify:
The valve requires 36,000 Nm to open, the actuator CAN produce 60,000 Nm. So therefore the valve will open.

It is a truism that an actuator will ONLY produce enough torque to move the ball/vane/plug, therefore in a perfect world once sufficient pressure is applied to the piston to overcome the spring force and the valve torque (36,000 Nm) then the valve will begin to open. So, for instance, if the actuator CAN produce 60,000 Nm when, say, 100 psi is applied it is likely that the valve will begin to open when, say, 60 psi of pressure develops in the cylinder and only 36,000 Nm of torque has been generated.

The problem will be if the valve siezes and more than 36,000 Nm is required. The actuator CAN produce 60,000 Nm and if the ball/vane/plug refuses to move it will do so if the required amount of pressure is allowed to develop in the cylinder. And that is where your problem starts as the MAST is 58,584 Nm. Once the actuator torque exceeds THAT level of output the ball/vane/plug may well stay put BUT your elastic limit of the stem will have been exceeded and you will have a twisted (or worse, broken) stem/key. The issue will be that any position monitoring device based on the position of the actuator drive will not be coincident with the position of the ball/vane/plug.

 

[gerhardl]

...and further to above excellent comments: you will also have exceeded any factory warranty. Using torque switches may limit the actual risk of deforming anything, but will leave any party 'unsure' if this limitation really has been working if anything happens.

Depending on 'working conditions' any valvestem should in principle in my opinion be able to take more than double the opening 'normal' torque.

 

[cam72]

Sini4s, Some additional info:
BS12490 clause no. 8.2 says “actuator maximum torque/ thrust based on the maximum operating pressure shall not exceed the valve MAST any point of travel”. If you ask valve vendor to comply to this clause, generally the answer is “actuator is fitted with safety relief valve, Air filter regulator and hence actuator will never see maximum operating pressure and hence torque values given are based on relief valve settings (which is generally 1 barg above min.supply pressure)”. If vendor tries to provide actuator torque values say at maximum operating pressure wo relief valve, actuator values definitely exceeds the MAST values and can create some problems for valve manufacturer ( may have to provide stems of higher strengths etc etc). Hence most of the time MAST values supplied by valve/actuator vendors are based on min. supply pressure.