The Effect of Changing the Steam Valve

In what context? What is the steam being used for?

The boiler generates steam for cleaning purpose for lances and to energize steam turbine.

I think they probably do it for lower cost. Globe valves can be used for adjusting pressure somewhat like a pressure regulator or needle valve. If you do not need to do this and just want to isolate the energy then a gate valve can do it alot cheaper.

pipewelder

<.....there is resistance flow thru the valve.......>

There is resistance to flow through all conduits and fittings placed in them. So what?

The globe valve will be a lot easier to maintain and seal against the boiler pressure. The gate valve does tend to accumulate sediment and it is difficult to seal steam lines completely as the seats slide together and are often scored, they are also difficult to repair and lapping the seating surfaces perfectly is almost impossible.

After thinking about this awhile I was wondering if you have a non-return valve in the steam outlet of your boiler? All of the boilers we have in the mill have a non-return valve which is basically a isolation valve and a check valve all in one package and another isolation gate valve with a way to bleed the pressure off in between. The non return valve prevents steam that has left the boiler from backing up in case you loose pressure or flow on your feedwater into the boiler and also acts as as one valve in our double block and bleed isolation system. I am not sure if you codes require a double block and bleed off isolation system but I would highly recommend one. I would at least be sure that you are not replacing one of these type valves with something that will not prevent backflow. This is just a suggestion but worthy of checking I think.

pipewelder

I´m not very familiar with those small boilers. In Power plants with a higher pressure and temperature we normally use for main steam stop valves parallel double slide gate valves. This have some advantages over globe or wedge type gate ones: They need much less force to actuating the valve (roughly half or less than a globe one). As said before all elements in a piping system produce a pressure loss (even straight pipe), but gate valves usually have a straight trough pass while globe ones have a "S" like pass producing for the same process parameters, more pressure losses.

Obviously those valves are used as "on -off" valves and in no way as flow modulating valves nor check (anti - return) valves. Gate valves are not the best suited for modulating service. If you need modulating, globe valves (needle included) are best suited.

Cheers

Gate valve is only used for on/off operations and can't be used for fluid controlling, where globe valve shall be used where such a controlling or a regulation is required.

In steam lines we used to use gate valve for on/off, since when the valve is fully open, the disk of valve will not obstruct the flow of condensates to avoid its accumulation at the bottom of line which causes what is termed by Slug Flow, which causes an induced forces and vibration. Even in case of gate valve is partially open, the disk shall not obstruct the flowing of condensate at the bottom of pipe.

If we installed a globe valve for only on/off, there will be a great drop in pressure and there is also a chance for condensate accumulation, otherwise a good trapping system is required.

Keep in mind the following:

1. What will the pressure drop be through the valve when fully open? You do not want to negatively impact the available pressure at the end of the steam line.

2. What are you using this valve to do? Isolation, on/off service, emergency bypass with throttling capabilities?

3. Where is the valve located? If it's the valve on your boiler, then that needs to be a crown valve (there are local boiler codes that will dictate the type of valve that can be used on the boiler, and for blowdown so don't change out the valves arbitrarily).

If the line size is relatively small (less than 6") and you just want on/off as an additional positive shut-off upstream of a modulating control valve, then an actuated FULL PORT ball valve may be your best solution.

However, if space is an issue you may have to use piston actuated valves.

You should still maintain manual isolation valves (globe, gate, or ball) in the system for safety purposes.

In a nut-shell - globe, gate & ball valves are suitable for steam applications - you just have to decide which one will work best for your system and are able to properly maintain with your existing maintenance department.