Steam blowing 3

[athomas236]

We are preparing a procedure for a boiler and steam pipes upstream of a steam turbine and would like to be sure that the conditions (velocity or velocity head) at the target plate are similar to those in the turbine. Does anyone how we can do this.

Any help appreciated.

Athomas236

 

[poetix99]

For a steam piping blowdown, the objective is to have some reasonable assurance that you've dislodged and discharged from the boiler and piping assembly any material that could damage the turbine internals during actual operation. (welding slag and sand blasting grit are great examples)

What is relevant is not to duplicate the steam velocities in the turbine, but to achieve velocities in the piping that exceed the expected maximum velocities in the piping during actual operation.

This does NOT require that the steam flow for the blowdown be equal to or greater than the maximum steam flow expected during operation. The blowdown will discharge to atmosphere, whereas the turbine inlet pressure is typically much greater and the velocities at the turbine inlet will be much lower; typcially < 200 fps is what I'm accustomed to.

You can calculate &quot;max normal&quot; operating steam piping velocities based on 250 fps at the turbine inlet. Then, calculate blowdown velocities based on the additional piping and the pressure drop during blowdown.

By the way, the normal steam piping is all that you need to worry about cleaning, but the &quot;temporary&quot; blowdown piping should be clean in the first place, so that you can decide from the target plate inspections that you are finished with the steam blows.

Ensure that any delicate instrumentation (flow meter primary element, pressure probes, ...) have been removed before the blowdown. Use temporary spool pieces if necessary.

There might well be a shock wave at the blowdown discharge, and the final pressure inside the pipe will be greater than atmospheric; something to better define when you calculate the blowdown velocities based on (boiler to discharge) delta p.

Blowdowns are very noisy. If you have neighbors, you will get their attention. You might want to warn them in advance of the &quot;one-time&quot; event.

Blowdowns are hazardous; noise and high velocity projectiles can be unpleasant. Absent any standards for guidance (I don't know of any, but I would be surprised if there aren't any) this should be treated much the same as the discharge of a full-flow relief valve with regard to safety considerations.

 

[MJCronin]

athomas....

Try

http://www.steamblow.com....

Have you considered calling in an independent contractor who specializes in this type of service ? Have you considered the merits, costs and schedule advantages of an &quot;air blow&quot; ? Is this a HRSG for a combined cycle Plant ?

There was a discussion of &quot;steam blowing&quot; in the &quot;Piping & Fluid mechanics forum

Tell us about the steam generator and the design conditions for the steam systems...

See the following websites.....

http://www.hosedragon.com

(Boyle Energy Services)

http://www.bisglobal.com

(very helpful, but chatty Englishmen)

Let us know how you make out and Good Luck

MJC

 

[athomas236]

Thank you gentlemen for your responses

I agree that the reason for steam blowing is to dislodge slag and scale from the boiler tubes and piping. For this subject our normal approach is to select a blowing pressure and flow rate so that the 0.5*rho*vel^2 in the boiler tubes and piping during blowing is at least 1.2 times that at boiler MCR. We do not consider velocity directly.

The issue for me is to decide when blowing has been satisfactorily completed and there appears to be a number of different approaches.

1. Use brass or aluminium target plates and continue blowing until the impact marks on the plates are not getting any better - the blow till it gets no better approach.

2. Same as 1. but with acceptance that blowing is satisfactory based on the number and size of impact marks are less than some limits - the blow to a criteria approach. With both these approaches I have never seen any recommendations about the velocity at the target plate, which should, I think, affect the size of marks on the plates.

3. Acceptance based on likely damage to turbine blades. Appears to be UK approach, so final target plates are blade material and velocity at the target plate is representative of those in the turbine - the blow till we will not damage the turbine approach.

Number 3 seems to have some logic (could be because I am English)whereas 1 and 2 seem to rely on experience and custom and practice, which is where my problem starts.

As a turnkey designer we have in the past prepared detailed procedures on other projects for the same turbine supplier as we have on this current project. On the previous project approach no. 2 was used with a boiler blowing pressure of not less than 50% of rated boiler pressure whereas now we are being offered approach no. 1 with a blowing pressure of about 20% of rated pressure. It is possible that this supplier has completely changed is standards in the last 6 months but unlikely.

To answer MJ questions the boilers are circulating fluidised bed boilers burning coal rated for 930000lb/h at 1950psig and 1000F located in central Europe. We have not considered an independent cleaning company and with blowing due to start after acid cleaning which itself starts on 17 Nov 02 there little time left to appoint to such a company.

For your information noise will be an issue as we still hve people living where we intend to build the gatehouse. We have a grave yard on one side and railway on the other so not too many complaintes expected.

athomas236

 

[athomas236]

Thanks for reponses, steam blow delayed now scheduled for 2nd week December. Discussions on procedure are on-going. Boiler acid clean starts today.
Regards,
ahthomas236

 

[C4Reactor]

Further question to the group.
I'm planning to do a steam blowing. Two set-up are being proposed:
1. To locate the sacrifical valve at the upstream side of the new steam piping, close to the steam source. Steam flow can be monitored from the supply header. Sacrifical valve is opened until desired steam flow is achieved.
2. To locate the sacrifical valve at the exhaust side of the new piping. Line will be pressure up until the normal operating pressure. Sacrifical valve will be opened to achived the target flow. Steam flow can be monitored from the supply header.

Target steam flow is 1.2X normal steam flow.

Appreciate any comment.

 

[athomas236]

The usual aim of steam blowing is to get a target disturbance factor that is at least 1.2 not to get a target steam flow that is 1.2 times normal flow.

Disturbance factor is discussed elsewhere in this thread but is basically the ratio of the velocity head during steam blowing to velocity head at boiler MCR.

Steam blowing usually takes place with pressures lower than rated pressure so disturbance factor more correctly includes the effect of steam density on the &quot;cleaning force&quot; for want of a better term.

The other advantage of using disturbance factor is that there is no need to measure the steam flow and take the risk of damaging the flow nozzle/orifice. This is because the distubance factor can be calculated by the ratio of the pressure drop over the supheater during blowing to that calculated at boiler MCR.

I have only ever seen steam blowing with the sacrificial valve as close as possible to the pipes to be blown. This seems to be for reasons of cost because (providing there is no other valve downstream of the sacrificial valve) the temporary steam blowing pipework can be designed for a lower pressure.

athomas236

If you are blowing at reduced pressures then the selection of the blowing pressure and size of the temporary pipe could require some calculation to be sure you can get the required disturbance factor.