Author Topic: A simple explanation of how a three element control works  (Read 1849 times)

Offline Joseph

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When I first heard of three element control, I just jammed as much as I could remember into the cerebral cortex as possible, dumped what ever I could later on the exam, and moved on - rather like refrigeration. Who remembers about refrigeration? The only reason I know about it is because I had to teach it - and, truth be told, I had to read up a bit to remind me what it was all about.

While working at one plant I finally was able to move up to the control panel and operate a boiler, and even though I didn't realize it at the time, the most important gauge on the panel was the water level. All I knew was to put the controller in auto with the right set point and move on to the next task.

The problem was I didn't really understand how it worked - and so I didn't really know what to do when something went wrong. Well, I'd figured out that if it went awry I could put it in manual and have someone stand at the bypass valve and open or control it until it was fixed but that was it. I did this once in Gold River, with the operator calling up on the radio and telling me "open (or close) it up just a bit..." and "ok, just a bit more" for a few hours until the instrument mechanic fixed it. It was a nice break for an hour or so, and then it was the most boring job on the planet. Kind of like manhole watch - oops, I mean person watch - on a confined space entryway procedure.

To understand how three element level control works, you have to understand why you need it in the first place. I mean, after all, why not just use something called 'one element control'? For example, if the level in the boiler starts to drop, put more water in. If it start to rise, cut back on it a bit. You would think that you would have one element to control - the water level. So there should be one thing to do about it - put more water in if it drops, or less if it rises. Simple.

But you can't do that. The problem with water level on a steam boiler is the level you see in the gauge glass isn't always accurate. Sure, some of the time it is, but something happens when someone - your customer, or a process in another building - takes a big steam draw.

When you drop the pressure quickly in a steam drum, the water starts to swell, and the water level rises. If you lower the pressure, you lower the boiler point. In other words, if you drop the pressure in the steam drum, then suddenly more of the water in the boiler starts to boil - suddenly a lot more little steam bubbles start to form - and the water in the boiler swells, and the water level rises.

You didn't get any more water in the boiler - in fact, as the water in boiler swells due to the pressure drop, the actual water in the boiler doesn't change - the mass of water doesn't change - but the level rises. Since the pressure drops in the steam drum, and the steam flow increases out of the boiler, you are actually losing water in the form of steam.

This is the problem with one element control. The pressure in the drum can drop suddenly, and just when you are losing water (in the form of steam) the water level rises. The one element control will look at the water level and decide that it should cut back on the water going to the boiler - exactly opposite of what you want.

When the pressure changes suddenly in a boiler, you can't trust the level. If you want to replace the water that is being lost, you have to do it another way.

It's like putting a hole in the boiler - all the steam starts to flow out of the boiler as the pressure drops. You're losing water. Sure it's water in the form of steam, and the volume of say a pound of steam is about 1700 times larger than a pound of water - but it's still water. In fact, you're losing pounds of steam, and therefore losing pounds of water.

If you can't trust the level, then how do you put water the right amount of water into the boiler?

First, consider this.

Imagine a bucket of water that is about half full, and it has a hole in the bottom. You want to keep it half full.

One way to do it is to measure the amount of water coming out of the leak, and put that much water back into the bucket. If you figure the bucket was leaking 3.2 liters/minute out the hole, then pour 3.2 litres/minute into it. If you think about it, you wouldn't even have to look at the level. You might peek into the bucket every so often after you had the flows matched up, and just slightly adjust the water going into the bucket so that the level is where you want it, just is case it wasn't quite right.

This is how a three element level control works. If the steam pressure increases or decreases because of a change in pressure - which means that the was a change in flow - then you measure the increase in the amount of water lost, and match it with pumping in that lost amount of water.

For example, we have a 150 psi View boiler in the lab that runs about 1200 lb/hr at minimum fire. If one of the other labs suddenly started to draw an extra 1000 lb/hr, then the three element level control would recognize that the flow of steam was now 2200 lb/hr, and would increase the flow of water by 1000 lb/hr and now add 2200 lb/hr of water to match the steam flow. The three element level control would prioritize the flows - steam flow out and the water flow in. The controller will "look" at the steam drum level after a bit and then top up or lower slightly the water level to match the setpoint.
The View boiler controller prioritizes these flows by about 70 percent, and allocates about 30% of the water level control to that actual water level of the boiler. This three element water level control - steam flow out, water flow in, and level - is how most plants control the water levels in their boilers.

Incidentally, most plants use a flow orifice to measure steam flow. As a result, during startup of a boiler you should keep your water level control in manual until you can actually trust the flow of steam coming out of the boiler.

The problem during start up is that the actual flow of steam is measured by the differential pressures across the flow orifice. In other words, the flow measurement device doesn't actually measure the flow, it measure the pressure differential across the flow orifice, and then it calculates the flow. If this flow is very low, most flow orifice devices have a difficult time measuring these low pressure differentials - the lower the differential, the worse the error in flow measurement. It has to do with the whole square root function calculation, which is a topic for another article. Suffice to say, you can't trust your steam flow measurement at low flows. You will get false readings of hundreds or thousands of lb/hr of steam flow when there is actually no steam flow at all.

This confuses the heck out of the three element level controller. If one of the elements input into the controller is false (steam flow), then the output will be false. What happens is the controller reads that the steam flow has increased, and so it increases the water control. Since the controller is biased towards operating by flow and paying a lower priority to the level in the steam drum... you could overflow your steam drum during startup. If things really go wrong, then the sound of that giant banging you hear is actually your piping being smacked by water hammer. If you see me running for the door, try and keep up. If you're brave, close the stop and check valve, open a couple of drains, and then see previous sentence.

Joseph
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Offline stewartforsyth

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Re: A simple explanation of how a three element control works
« Reply #1 on: May 07, 2016, 09:42 »
Thanks Joseph a well written explanation, that why is so helpful.  :)
Stewart
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Offline Colin Farquhar

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Re: A simple explanation of how a three element control works
« Reply #2 on: May 12, 2016, 13:09 »
Thanks for the info-always good to hear a different explanation beyond the PG texts.  It also helps to draw parallels between what we do unconsciously when firing a manual boiler, and what the automation is up to on a more modern unit.

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