Chattering in Quick Closing Cold Reheat NRV (CRH QCNRV)

[Sanjay Kr]

Dear Experts,

We are facing chattering issue in our QCNRV. It's installed in steam turbine's CRH line. The fluid media is saturated Steam at 55 Ksc and 365 deg C Temprature. The size & other details are as below:

Size of Valve; 34"; 900 Class
Valve Type: Quick Closing NRV
Material : A217 WC6
Flow: 966 TPH
Pr. Drop: 0.2 Ksc
Velocity: 26 m/s
Fluid: Saturated Steam
Upstream Piping: 5 Mtrs straight pipe length
Downstream: 190 mmm & afterwards T is present.

These are two valves placed in parallel at 1800 mm apart and are connected to common header in downstream at 190 mm with Tee.

Please note that we are observing lot of chattering in these valves and due to this the NRV hinge is getting damaged regularly. Request you to share your views to counter this problem.

Best Regards
SK

 

[hacksaw]

A check valve in a high pressure steam line? Are you certain about you process design?

 

[LittleInch]

#impossible for us to understand without some piping and process information.

A section view of the valve would help

What position is this tee - at 26 m/sec sounds like you're getting resonance and velocity too high.

What does valve supplier say?

Is there a velocity limit?

Is the 26m/sec in the line with the valve in or in the header?

When is this occurring in the operation - At high flow/ low flow/ no flow?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hacksaw]

LI, do you find it a bit unsual to have a Size 34 NPT check valve in a Class 900 steam line with 190mm discharge piping?

Never seen HP steam piping like that.

 

[LittleInch]

I think he means 190 mm downstream of the 34" valve there is a tee where this valve is the branch line (so header must be really quite big.)

Sounds like two lines 1800mm apart feeding into a header.

But this is why we need a decent drawing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Presume these check valves are not installed on a vertical run with flow going upward - that can a possible cuase for chatter. What type are these check valves - dual plate or swing or some other special design? How is it made to be quick closing?
Units for press drop are not familiar - Ksc?
What is CRH line ?

 

[LittleInch]

Think it's shorthand for kg/cm2 (Kilograms per Square Centimetre)

CRH looks like cold re heat.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[stgrme]

Cold reheat (CRH) check valves are required for large reheat turbines with an HP turbine bypass system. See the typical outline image file enclosed. The check valves prevent back flow into the HP turbine exhaust during start-up when CRH pressure is greater than HP turbine exhaust pressure. As the start-up continues, steam is admitted to the HP turbine, the HP turbine bypass valves are closed slowly and the HP turbine exhaust pressure begins to rise. I suspect that the chattering of the CRH check valves occurs when the differential pressure across the check valves is small.

Each check valve is equipped with an external arm with a counterweight. The counterweight is normally adjusted so the force of the check valve's disc (clapper) on its seat is balanced (zero force) or slightly positive. In addition, some check valves are also equipped with an air cylinder and spring. The spring provides a positive closing force on the disc. When air is admitted to the cylinder, the spring is compressed allowing the disc to move freely and open as pressure and flow from the HP turbine exhaust increase. A solenoid valve on the air cylinder exhausts the air in the cylinder to atmosphere when it is energized, allowing the spring to assist in closure of the check valve. Please note that the spring force is not sufficient to overcome forward flow through the check valve.

I suggest the you check the adjustment of the counterweight to provide a slight positive force of the disc on its seat. If the check valves are equipped with air cylinders and springs, I suggest that you check the settings and operation of springs and solenoid valves.

Best of luck!

 

[LittleInch]

We don't know if this occuring at low DP / low flow through the valve or low Dp/ high flow.

In reality we know virtually nothing about this particular situation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

From the net, it looks like these are power assisted swing check valves. Doesnt look like there are any special features to eliminate chatter at low flow when these are energised to open, particularly when installed on the vertical with upflow.

 

[hacksaw]

"Size of Valve; 34"; 900 Class
Valve Type: Quick Closing NRV
Material : A217 WC6
Flow: 966 TPH
Pr. Drop: 0.2 Ksc
Velocity: 26 m/s
Fluid: Saturated Steam
Upstream Piping: 5 Mtrs straight pipe length
Downstream: 190 mmm & afterwards T is present.

These are two valves placed in parallel at 1800 mm apart and are connected to common header in downstream at 190 mm with Tee."

as LI points out, a layout drawing would clarify the inconsistencies

 

[davefitz]

It is possible that the power actuated assist mechanism has been designed to dampen the final 3% closure to prevent steam hammer, and that adjustment was not calibrated properly. Also, if power assisted, then the timing of the power adjustment vis a vis the opening instant of the HP bypass valve is not correct.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[LittleInch]

Much depends on the type and set up of the valve.

This video is quite enlightening

https://www.youtube.com/watch?v=rIopHFosSr8

and this description

http://www.irimex.kz/files/catalog/files/r_1776_Check Valves Side Mounted extraction check valve.pdf

where interestingly it says the alst few degrees is often left free to swing as opposed to the spring forcing it closed until a certail (low) DP is present.

So chattering could be from many reasons.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[stgrme]

I re-read the original posting from the OP. In my initial reply, I suggested that the disc (clapper) might be operating close to its closed position. However, according to the OP, damage was seen on the hinge. If the disc were operating close to its seat. I would have expected damage to the sealing area of the disc and the seat. Damage to the hinge now leads me to believe that the check valve is not fully open (disc in contact with backstop in the valve body).

Although some respondents have indicated that a velocity of 26 m/s (85.3 ft/s) seems high, it may not be sufficient to fully open the check valve. According to a “rule of thumb” from one manufacturer, the minimum velocity (in ft/s) to fully open some of their valves is six times the nominal diameter of the check valve. Also note that velocities up to 150 ft/s (45.7 m/s) in CRH piping systems are not unusual.

I suggest that you contact the valve supplier to determine the minimum velocity in the check valve to ensure full opening. Be sure to ask the supplier if the velocity is based on the nominal diameter of the check valve or on the diameter of the seat. If based on the seat diameter, remember to ask the supplier for the seat diameter since many of these check valves have reduced-size seats.

I also noticed an inconsistency in the data for the check valves. According to the original posting, the fluid media is saturated steam at 55 kg/cm^2 and 365 deg-C. However, the saturated steam temperature at 56 kg/cm^2 absolute (55 kg/cm^2 gauge) is 270 deg-C. Therefore, 365 deg-C must be a design temperature for the check valve rather than an operating temperature. Please check the operating conditions for the valves.
Best of luck!

 

[georgeverghese]

The chatter of a swing check valve 24/7 at low flow on a N2 line to a sphere for months on end still rings in my head sometimes. But this check valve never failed mechanically, even after 12-18months of non stop chatter. Since this is HP steam, given this mechanical damage on the swing check internal hinge, I'd suspect there is some corrosion due to impurities in the HP steam or galvanic corrosion from dissimilar materials in the hinge assembly.

 

[hacksaw]

Saturated?, gee maybe ha needs steam traps to eliminate slugging...maybe when the OP gets back he can clarify all the issues raised.