Energy losses in steam piping

sounds like you have some impingment points in the piping.

you may have to put in some steam traps at these points

phoenix911

How about the insulation on the line?

The piping is pretty well insulated, so i'm not so worried about temperature losses. I guess the thing that puzzles me is that when you lose 50 psi steam pressure because piping and orifice are way too small, what is the steam quality at the discharge? Is enthalpy conserved and so the steam is superheated, or is the loss in pressure all converted to velocity head, or is some converted to heat by friction, or some of it lost as sound?

Feel like I should know this after all this time....

The easiest way to answer your question is to do a temperature/pressure survey on the length of pipe under question. Actual field measurements if they have not been done yet!

Figure out where the pressure drop is actually occurring. Maybe you find it's not the control valve.

T/P readings on the downstream side of the orfice plate can be used to determine the quality of steam via steam table. You may need to apply some tricks to get T due to insulated pipes.

Enthalpy will be conserved across a throttle however that doesn't mean it is superheated.

Hope this helps!

-David

I have been racking my brain on how best to answer your questions, here are some points I came up with:

1. Please review the paper I placed at this site:http://ifile.it/pajeo0y
2. Yes, enthalpy is conserved - and with pressure drop calculations for steam flow the process is called isothermal pressure drop because a constant temperature ideal gas expansion also requires constant enthalpy.
3. The majority of the energy lose is due to friction, particularly at that high of a velocity in the 4" pipe (285 ft/s) normally you don't want to venture any more than 200 ft/s and in your case you should probably go with around the 125-150 ft/s range.
   

It is a rule of thumb that increasing the pipe size will reduce the friction loss (drop of pressure) and will reduce the power for steam pumping (lower pump head). Also, reducing the velocity will reduce the friction loss. Also, try to obtain 1-phase flow (only steam) by discharging the condensed water by using proper steam traps with well designed pipe legs. In addition to checking the hot insulation w.r.t its type, thickness and damaged areas.

No, it will be wet steam, not superheated. Energy is always conserved. Steam has the property of condensing readily on colder surfaces.

Sort of depends on what you do with that steam. If going to feed a turbine, you'd better make some changes. If for heating you are ok if the temperature at the point of use is ok for you. The pressure drop is lowing the condensing temperature (see steam table). I would suggest you ditch the orifice and put in an annubar or better a vortex shedding meter (more accurate). Make certain that the pipeline is properly insulated. A measure of the flow rate from your steam traps will give a pretty good measure of the energy losss.

profsteam

Thanks for responding, I did read the Guests' article and it clarified some concepts regarding heat lost from friction, pressure lost due to acceleration/expansion of steam. But, still i don't have a clear picture of what's happening with our porblem.

Okay, here's a little more information that will hopefully clarify my question. We have 8 digesters in a paper mill, big vessels that are filled with wood chips and sodium sulphite and then heated with steam to "cook" the chips. 4 of these digesters are fed with 150 feet of 8" piping, an orifice plate and a control valve, and 4 are fed with 4" piping, an orifice plate and a control valve. All 8 pipes start at a big steam header that operates at 135 psig, saturated steam. All 8 digesters start out at 30,000 pph steam flow and taper off as chip temperature rises to final temp.

I know that the pressure losses are higher in the 4" pipe than the 8" pipe, and higher in the 4" orifice than the 8" orifice. However, the control valve on the 4" systems STILL CONTROL FLOW TO THE DIGESTERS even at 30,000 pph. In other words, the 4" systems still seek to remove pressure/flow from the steam supply before the digester. You would think that since both systems are in control, then we don't have a problem. But, the digesters fed by the 8" piping COOK FASTER than the digesters fed by the 4" PIPING. Cooking time is directly related to temperature of the wood chips, and is approximately 45 minutes.

So, the question is, if we're still in control on both systems, what is the difference between the two? The only thing I can think of is the steam quality is different at the digesters fed by the 4" piping because energy is lost from the steam in the piping. But, if that's true, where did it go? Is it a simple as friction lost to heat and radiated off insulation, and/or sound energy lost at the orifice?

We'll do some temperature and pressure measurements this week to see what we can see. Thanks again for the feedback....

Check the calibration on all your flow meters carefully. Have you tried taking steam calorimeter readings at the supply connection to each of the digesters? Is the steam flow to each digester modulated by measuring the digester batch temperature?

profsteam

I am assuming that all digestor or running at around 85 Psig and steam is introduced in the digester at this pressure.

If we assume that the flow is indeed 30,000 LBS/Hr flowing thru either 4" or 8" line and delivered at around 85 psig to the digester you are providing the same energy to all digestor,and all should heat up in the same time.

I don't think 30,000 Lbs/hr is flowing thru the 4" line.

You are NOT allowed to drop 32 PSID across the orifice when inlet pressure is 150 PSIA.P2/P1 should be < 0.75.You may not be reading the correct flow.

Even if we disregard this rule what we have for the control valve pressure drop is 135-32-85=18 PSI,assuming you are introducing 85 psig steam to the digestor.

To handle the flow of 30000 LBS/hr with pressure drop of 18 psi you need a Cv of approximatley 236 which is the cv of 4" valve.

Is the valve full open when 30,000 lbs/hr or almost full open?.

Yes, I agree - I think the problems lies with your orifice flow meter:

1. When was the last time the orifice was checked? Steam flow (especially 2 phase) is a harsh service - particularly at the velocities you are dealing with - the orifice may be washed out and your measurement is wrong.
2. Depending on where the orifice is at to which pressure it is calculated - your measurement may be as much as 20% off. The reason being that the orifice was designed/calibrated to 135 psi, but due to pressure drops in the line, the actual inlet pressure is going to be lower.
3. Confirm the design conditions of the orifice.
   

I don't understand why you have a flow control on this process anyway. Why not just have an on/off valve on each line? Then the steam delivered will be at the highest possible temperature given the line sizes and give the maximum heat transfer rate which seems to be the ultimate goal. As the batch heats up the steam flow will automatically adjust itself to lower and lower steam flow rates until equilibrium is achieved. When the batch is done, turn off the steam valve.

profsteam

Your 4 " pipeline is drastically undersized resulting in excessive increase in velocity and resulting in pressure drop. Steam pipelkines are designed on p1-p2/length. p1 = initial pressure, p2 is final pressure that you want & length = 150 feet. Ideal velocities to be within 80 ft - 120 ft/sec.Saturated steam pressure is interrealated with temperature & vice versa. A drop in pressure = a drop in temperature & vice versa. This delays heating & processing time and increases boiler operation time hence, excessive fuel wastage & HIGH bills.

The Steam mains to be designed to run at slope in direction of flow. equal diameter drain pockets to be installed at every 100-120ft tapped with engineered drain traps.

Lastly efficiently insulated & jackated.

I doubt your steam flow meter for it has been calibrated by the Supplier to read 30,000 lbs/hr. You can never get an accurate reading unless it is density-compensated.