MOV not opening fully 1

[nesamani1983]

Dear All,

Refer the attached dwg.

During the pump starting, as per the starting sequence(defined), when speed reaches the set point, discharge MOV has to open.Total MOV opening time is 35 secs. But in pumps-B & C , MOV-11 & 12 doesnt open fully. MOV stops in the half open position.Ball valve is operated by MOV.
This problem occurs only during no other pumps are running, means P1 = P2. If any one pump is running (P2 > P1) , MOV opens smoothly.
Note: (1) This problem has started after the overhauling of respective the pumps. Pump-A is not yet overhauled.
(2) P1 <20 Kg/Cm2, P2 = 50 to 70 Kg/Cm2 if pump is running.
P1 = P2 = 25 to 30 if pump is not running.
(3) If pumps are not running, means low flow in pipeline, then flow will go thru pump by-pass line (MOV-4).

Kindly give me the solution...

 

[NGiLuzzu]

nesamani1983,
just a first, quick trial to explain what MAY have happened: pumps overhauling may have introduced into the disharge valves something (dirt? moisture?) that increases the friction between ball and seat, so that - at a certain point of the stroke (half open position) - the electric actuator can't fullfill the valve torque requirement anymore; probably a differential pressure across the valve helps to win such a resistance somehow...
This provided that the actuators work properly (I assumed You already checked them), of course.

I'm just trying to guess... but please let me know if I guessed right, in the end ;-)

Hope this helps, 'NGL
________________________________

 

[gerhardl]

As anegri states the import question is: what have changed?

From my lifelong practise you usually have two main points creating actuator problems:

1. Startup and
2. after main maintenance on pipelines.

Point 1 includes mainly false mounting, adjustment and connections.

Point 2 mainly includes dirt or impurities/residues/dried deposits/growth or even left mechanical things inside pipeline (Rest from welding, boring or grinding, tools, T-shirts, cleaningcloth etc) but both points including problems from the other.

1. Agree with anegri: torque has increased as indicated by Point 2 above, on ball, sealings or in pipeline. Ball and sealings might be damaged.

Demounting and inspection necessary. Change complete valve, ball or sealings and/or clean.

2. As valve is operating normally at times, I presume that all mechanical items including the el. operation is Ok, no damaged limit switches, mechanical gearing or other weakening or hindering or slack in transferring the mechanical torque to the ball?

Recheck.

3. Electrical check including instrumentation.

4. Torque will normally increase somewhat over time, and for ballvalves the actual torque could well be above the expected 'normal' initial one to two times.

a) Check torque switch setting (if torque switches) and increase if possible (risk of damaging ball-valve (mechanical and sealings, spindle and spindle fastening OK for this?) or actuator/gearing.

b) If time of closing and opening might be increased (check if control system is programmed with time limits), torque may be increased by adding an extra gearcase between existing actuator and existing gearcase or valve if mounted directly). Same risk as for a.

Above should be done in sequence (3,2,1 and 4) with recheck before 4 if serviced/repaired on other points.

 

[hydromarine]

Have seen this after refit were the MOV coulnt open a Grove ball valve stripdown and inspection found a 1/4 bolt stoppig the ball from opening on the seat area ,we intially thoght it was the italian MOV causing problem