Control Valve Cutting

[ly1phil]

I'm trying to replace/modify a series of valves that reduces pressure from 1600 psi to 400 psi. There is currently one PRV along with an isolation valve in series. Both get steam/condesate cut in a matter of months. These valves are cycled approx. twice a day.

Will replacing the isolation valve with another PRV help the situation? This would allow two pressure drops across two valves from 1600 to 400 psi.

Also, does steam cut valves when they are cycled or condesate or both?

Thanks for the input.

 

[insult2injury]

What exactly do you mean by cut? Is the trim damaged/failing? What are the materials of construction?

How do you plan to control the two valves in series?

Typically, a two phase mixture at the inlet to a control valve is a no-no and should be avoided. Perhaps you could provide a bit more detail on the application.

I2I

 

[ly1phil]

I've only seen the valve repaired once, and at that time the valve seat had developed a groove (it is a high pressure control globe valve). Both the seat and plug is 416 SS.

I would use a split range controller to control both valves.

There is approx. 16 ft of vertical 2 inch pipe down from the steam drum to the control valves. The steam drum contains 1600 psi sat steam. These valves are used twice a day for about an hour. While the valves are closed, I assume that the 2 inch vertical pipe begins to fill with condensate. There is no steam trap upstream of the valves.

Thanks again.

 

[JLSeagull]

Wire drawing can be a common problem when operating near the seat. I saw this on small steam condensate and hot sour water boot valves. If the valves often operate near the seat, consider changing to a gap control that closes the valve below about 15% output. This can reduce the operation with the plug near the seat thus generating very high velocities.

 

[insult2injury]

Sounds like a blowdown valve. If it is, the inlet to the valve should always be condensate (nozzle below the low water level). The flow flashes in the valve as the pressure is reduced. Often these valves are multistage (cascade geometry) angle style valves and are designed specifically for the service. Depending on the service conditions, other valve geometries/trim styles are suitable as well. What do you have installed? Stellite, or other hard face material, overlay on the seat/plug may be required

If this is a blowdown valve, I definitely would not recommend using two valves. I think your best solution is to purchase a valve suitable for the operating conditions. The service conditions are not unusual and a competent valve supplier should be able to get you the right valve provided he's given the correct information to do so.

I2I

 

[ly1phil]

Actually it is a sootblowing supply valve, not a blowdown valve. The inlet isn't always condensate, under normal operating conditions the inlet is 1600 psi sat steam.

Like JLSeagull suggested, wire drawing may be the issue. The valve could possibly be over sized.

Thanks for the input.

 

[insult2injury]

For a properly design/located sootblowing valve, there shouldn't ever be condensate flowing through the valve. The steam actually becomes superheated during the pressure reduction.

It's very likely that the failure is a result of wire drawing/improper valve sizing. You may be able to just change out the trim (different design/materials) without buying a complete new valve. Talk to the OEM for guidance on this. The use of a hard faced overlay will help resist the wire drawing; however, it will still fail eventually. If the valve is mis-sized, correct that problem first.

I2I

 

[athomas236]

Sounds to me that your problem is in the design of the inlet piping and its drains.

Firs, I would talk to the sootblower supplier because they usually have guidelines about the design of inlet piping to prevent damage to the valve. Such guidelines can include ensuring that the common sootblower supply pipe vertical with a steam trap at the botoom to clear the condensate and the steam supplies to individual blowers branch off horizontally from the main with the lowest branch being 1m from the ssteam trap.

Regards,

athomas236

 

[djack77494]

By the way, do NOT use a split range controller. I would describe the service fully to your control valve supplier. He/she should be very familiar with this type of problem and should have a suitable line of valves for it.