Steam Velocities in Power Piping

[mb73]

I am new to this forum and just wanted to start a discussion on allowable steam flow velocities in reheat steam piping.
Crane No.410 gives an acceptable upper limit as 20,000 fpm. However, I am interested in the basis for this. Is it because of the pressure drop associated with this, noise, shock formation, scale removal,creation of high vortices around thermocouples in the piping???

And secondly, how reactive is the IP turbine to pressure drops beyond design.? For example, if at full load, the reheat stop valves see 500 psig and there is an increase in upstream piping size resulting in a pressure of 490 e.g. how would that impact turbine performance?

Looking forward to your responses.

 

[TD2K]

I would suspect this 'limit' comes out of pressure drops.

20,000 fpm is 333 ft/sec, your dP is going to kill you except on short runs.

At these velocities, noise will be another issue but I would guess you'd have noise problems long before this velocity.

 

[poetix99]

I agree (again) with TD2K. High pressure drops, lots of noise,...

Crane TP410 is a great resource (its been cited zillions of times in these forums), but is not really a piping design reference.

In typical power generation applications, steam velocities would not exceed 150-180 fps at "normal" or guarantee, and 200 fps at maximum.

Quibble if you wish by 10% or 20%, but the point remains that 20000 fpm is higher than any designer should design for in a power plant.

 

[hacksaw]

mb73,

Agreed, Crane 410 is the wrong starting point for your questions.

Reheat lines are a tough service because of the thermal shocks, etc. You need to start with as steam plant handbook. B&W has a good one: "Steam", but there are others.

Any intrusive pipe fitting is going to see enormous flow stress. Even a 1" thermowell (4" long) will see a static 50 lbf trying to bend it. The resulting stresses should resonace be excited can be >the maximum allowable stress for the material.

 

[mb73]

Thanks all. Good points. I found out that a turbine manufacturer stipulated not more that 12,000 feet/min as entry conditions to the reheat stop valve.

As an additional qustion, have you seen reheat lines at the turbine with a flange? For example the U-bend that comes to the top of the turbine shell to supply steam to the top of the IP shell usually is cut by mechanics, to allow access to shell removal etc.
I was wondering whether we can flange that section to allow less time for removal. Of course the thickness of the flange and the allowable turbine loads would play a very limiting factor.

 

[hacksaw]

mb

it is a matter of thermal cycling.

We had a swing boiler that normally sat at idle. The flanges in one loop (for a flow meter 10" 600#) were a continual maintenance item.

The were cut out and a meter tube welded inplace. Maintenace free now.

 

[25362]

Wet steam may be corrosive/erosive at high velocities. In particular to carbon steel piping when the flow is turbulent, and the moisture has a low pH because of dissolved CO2 or other acid-forming ions present.

The other extreme would be carbon steel cracking by Na traces brought in by injections of de-ionized water to superheated steam. Na converts to solid NaOH on water vaporization. But, of course, this isn't connected with linear velocities.

 

[davefitz]

The sizing of the reheater piping is soley based on limiting the pressure loss. The pressure loss has a very stron effect on the steam turbine heat rate /cycle efficiency.

In the old days, a 10% loss on the piping and a 10% loss in the boiler reheater was acceptable, but now these are reduced to about 5% on piping and 7% in the boiler reheater.

One would apply a 2% loss to the cold reheat and 3% to the hot reheat, and adjsut to meet standard pipe sizes.

If the unit is very large ( above 600 MWe), then it is likely that the pipe size would exceed 36 " NPS, which is the upper end for seamless hot reheat piping. If a rolled and longitundinally welded pipe is required, moden practice is to use a 60% weld strength reduction factor to avoid the Mohave syndrome.

 

[poetix99]

davefitz,

What is the "Mohave syndrome" (or perhaps "Mojave&quot?