Questions for you engineering type guys

Steve Ebels_3

A little background first.

We installed a Vitodens for a customer who turns out to be a facilities engineer in a manufacturing plant near here. He was grilling me during the install about why we did this that and the other thing with our piping since it was so different than what was previously installed in his house. In short what we did cured all of the problems that he had been experiencing with his system. A few weeks later he invited me to the factory to look at a system which provided 250* water for a process in the plant. They had issues with temperature control which were mainly due to incorrect piping, air elimination and to some extent the way the process was controlled. Evidently they appreciated what I told them because they have now asked me to look over a set of prints for a new plant being constructed in North Carolina which is to be driven by a 500HP steamer supplying 300* water to their press through a honkin' shell and tube HX.

A cursory look at the plan shows flow rate of 1200 GPM through 8" steel going to the press. The process involves taking the press, which is basically 100,000 pounds of steel, from 80* up to 290* in steps, over a 20 minute time period. I'm wondering if they have the capability for this in all the system components because they are working with only a 10* temp differential, water temp to product. No mention is made on the plans of what steam pressure/temperature is going into side A of the HX.

It would help to know the difference (specific heat?) of steel vs water so I can calculate the btu requirements of this load.

There are a number of devices drawn in the plan for water treatment on the steam side including RO machines, deareators, filters, water softeners and chemical treatment. What is typical?

I'm calculating roughly 8' per second flow velocity and 2.2 ft of head using 8" steel and 1200 gpm. This is just pipe, no fittings taken into account. Do these numbers sound accurate?

Thanks in advance

ALH_4

pipe

For materials information, Matweb is a nice resource. Just do a search for your alloy and they give you nearly every property you could ever need.

I get 7.66 ft/s velocity in 8" pipe at 1200gpm. At that flow rate and pipe size, I get 2.24 feet of head loss per 100 feet using Hazen-Williams or 2.06 feet per 100 feet using Darcy-Weisbach and Haaland.

What you have said sounds pretty reasonable to me, depending on how much it will be run. Generally, I like to keep velocities below 6 ft/s, but head loss does not appear to be a problem with 8" pipe.

bob_46

Specific heat

of steel .118 Btu/Lb/F*.

Steve Ebels_3

'nuther question

On the plan, the engineer is controlling the Side B water temp via motorized ball valves which regulate the amount of steam going into side A of the HX. In other facilities that I have been in, the water temp was controlled by motorized mix valves on the water side of the HX, blending return water back into the mix to regulate the temp. Which is more preferable?

To me, the ideal would be a VFD injection circ on the water side of the equation which would allow much more consistent and quicker control of the water temp.

bob_46

Steve

Not an authority or eng. All the steam to water heat-x that I have installed controled water temp by throttling the steam. I have never seen a ball valve used to control steam. Usually a double seated steam control valve. Pay close attention to how you handle the condensate.

Brad White_9

Steam Control vs. Water Control

Steve, I agree with you, for precision, water blending control is prefereable. Steam control to maintain a fixed your gross primary temperature is fine. but to make it it's best and depending on the load, the control valves should be set up 1/3-2/3.

A smaller valve you can imagine is far more precise than a larger one. For tighter control, I will use a set-up of 1/3-2/3-10%. That last 10% valve may even be a 1/2" valve with a reduced port. Great for small loads or initial system warm-up. Remember this is on the higher temperature water system side. The secondary side is far better at compensating for the final process.

This gross water temperature is then mixed down on the water side for superior control. I have used this in a chocolate plant for holding and melt-tanks.

Remember, with steam you are controlling a compressible gas versus an essentially incompressible liquid. This much you have assumed already.

For the ball valves as steam control valves (or water for that matter), I use "characterised ball valves". There is a groove cut into them that allows us to call them by another name: Camel Toe Valves. Much more linear.

Jamie Hall

ball valves

I've never been real happy with ball valves as throttling gadgets. Bob mentioned the double seated steam valves (in effect, a form of double seat globe valve; inherently balanced and excellent controllability) and Brad mentioned the camel's toe variety of ball valve, which also works. If you are throttling saturated steam, you need to be just a little careful -- but you seem to have ample pressure. I'd rather mix the water...

Unknown

nice link andrew, thanks!

PJO_5

Treatment Side...

Steve,

I have some experience w/ RO's and the like - in fact I am in charge of three such systems in my current job.

I agree with the control for steam. One system here's uses a ball valve and it's "jumpy" (keeps throttling up and down) on the 1.5" feed line for a 180F water system. Another system uses a valve similar to what they describe and it stays nice and comfy within 2F most of the time...until somebody decides to fiddle with it! ;-)

Send me an e-mail, and maybe I can help.

Take Care, PJO

Perry_3

Water Treatment and Steam Control

What you have desribed for water treatment prior to the boiler is quite standard in the industrial world. What exactly is neeeded for water treatment depends on the source water - and the pressure of the boiler. A very typical system would use, in order, a filter, watersoftner, chemical injection to kill the chlorine, RO, airstripper, and then perhaps Deminerilzer bottles (depending on how pure they wanted the water).

A very common option would be a green sand filter to take out Iron from the water - and would usually be placed just before or after the water softner.

They will also commonly inject chemicals into the boiler feed line to optimize boiler chemistry (and even entire steam cycle chemistry - you can control the PH in the HX and in the condensate with boiler feed chemicals).

They may well have a steam deareator/storage tank as part of the condensate return or boiler feed (very smart move).

Thus, what you have described is very common. I think it would be your best bet to ask why they are doing what they are doing so you can learn (and explain that you are not an expert on boilers of that size nor common industrial water treatment - and would like to learn what you can).

I will point out a simple rule of thumb on boiler water:
There ain't no such thing as too clean water and
proper chemistry - but there sure are many cases
of not clean enough water and bad chemistry.

As far as heating the water in the HX and steam vs water control. Steam control is very common and usually works very well. I do not like ball valves - and much perfer proper throttle valves. The type described by others is common, but their are many variations on cage assemblies and how they work. With the right throttle valve I see no problems controlling the water temperature fine enough by controlling the steam temp.

If it was me desinging this system (or checking it) - I would contact the control valve Mfr specified - or go to one of the better control valve companies (Fisher, Bailey, Masionilie, and a few others) and ask them which vave and trim they would specify for the application. If their is uncertainty get a valve with changable trim (the guts of the valve) so that you can change the trim if it doesn't work out. Such a valve is more money than some - but they sure do save a lot of time and money when it comes to tuning a system (and power plants are full of them).

Hope that helps,

Perry

Perry_3

Forgot an item.

For water treatment - it is not uncommon to see a large activated carbon filter. That would be prior to the RO.

If the water source has specific things their may be other filters or treatments to deal with those.

Perry