PID control usage

My application is a pump station:



Than water reached Lead Level – pump start with 100% speed.

Than discharge flow reached 25% - pump speed reduced to 60%.

If water level still increasing – pump speed increasing eventually up to 100%.



I’ve implemented logic:



START_LEAD < LEVEL & FLOW < 25% - SPEED = 100%

START_LEAD < LEVEL & FLOW > 25% - SPEED = 60%

L_70% < LEVEL < L_80% - SPEED = 70%

L_80% < LEVEL < L_90% - SPEED = 80%

L_90% < LEVEL < START_LAG - SPEED = 90%

START_LAG < LEVEL - SPEED = 100%



I’ve tested this, it works well for me. But my boss and another colleague who is a young graduate tell me that I have to use PID control. I’ve learnt carefully chapter 8 in DL405 manual and don’t understand: Why and How I can use the PID in my application.



Can anybody tell me what I am missing?

Thanks.

What ever you have done is corrct.
look its based on Lookup table

But for getting accurate and correct system you need to add PID controller
For this case you need to give Level Output as PID PV (Process Variable) and based on PID CV (Control Variable ) you need to Start/ Stop Pumps

Pls conform The Pump is have VFDs ?
What are ratings?

Thanks
Sandymax

I think what they are saying is that with PID you could get it to a "setpoint" (a certain level) and maitain at that level.

Originally I thought what you did was OK but it seems that your application is very common for PID use, even the DL05 manual uses a water level application for its example.

What PID does is take something to a predefined setpoint and maintain it at that point...ie 350 degrees in an oven, cruise control set on 70....water level at XX level. Instead of the motor having to constantly vary the idea is to have it reach a level and maintain that point as close as possible.

I assume you are using a VFD and analog inputs for the water level. The PID and drives people should comment sometime today.

Alex Pel said:

But my boss and another colleague who is a young graduate tell me that I have to use PID control.

Click to expand...

That's because all the other kids use PID, and because PID has these wonderful cool equations that the recent grad studied in college, and because PID looks really neat in the software. Using PID may also generate a lot of service income when you have to go out and re-tune the loops when operating conditions change.

Of course the project specifications may call for PID control, and that is because of all the above reasons as well.

I have been doing a lot of very sophisticated process control for years, and I don't use PID for doing it. PID isn't mandatory for analog process control, but it's "the way we do it" and a lot of people don't really understand why and when it is the most elegant technical solution (or why and when it ISN'T).

If your objective is to maintain a constant level in your wet well or in a discharge stand pipe, then PID is a common and reasonable technique. If your objective is to keep the water level in a raw sewage wet well between the limits of backing up and surcharging the sewers and to keep the pumps from running the wet well dry, then your logic will work just as well and is a lot simpler for the operators to work with. Not to mention that tuning a PID loop is a non-trivial task which you have avoided.

Get some more info on WHY they want to use PID. If it is just because "that's how we do it" you could point out that doing something because you have always done it isn't engineering, it's superstition.

You should also remember, though, that the boss isn't always right, but he's always the boss!

PID great BUT ,,,,,,,

The question I have is this
What is wrong with float switches?
PID will control the tank level to within a half inch (assume).
Float switches can control to plus or minus 6 inches.
What is wrong with that??
As long as tank does not overflow, and it does not go empty and burn out pump then all is good. What are the added risks, lowered maintainablity of more complicated controls when they may not be needed?

Granted my brothers have called (and still do) me the King of Overkill and YES I do tend to make things more complicted, BUT I have few callbacks and have not burned any houses down - or anything else.

I think we in America are too fascinated and locked in to electronic control, solid state, automatic, state of the art, etc etc. Are we putting in complicated controls and controlling to the point of ridiculous??

I dont know but then that is why I am going to school.

Dan Bentler

I don't think his boss is 'worried' about maintaining a level, rather he's looking for a 'constant'discharge flow down-line more.

Another aspect to look at is energy use and efficiency. We use electronic/automation devices to make the process more consistent and efficient which in turn may save on energy costs.

I personally havent done much with PID in a PLC. I can usually do something like the posters original idea and it work just as well for the application.

The application may not need PID or it may be more efficient with it ...I dont know. The boss is always the boss though and his decision is usually what has to be done.

Aquadevel said:

I don't think his boss is 'worried' about maintaining a level, rather he's looking for a 'constant'discharge flow down-line more.

Click to expand...

If he is looking for constant flow then he needs a flow transmitter instead of a level transmitter for his control. In that case PID would probably be appropriate.

I'm going to guess that this is not what he is looking for. Most pump stations on sanitary sewer systems have wet wells and are worried about flooding or pumps running dry - the flow in is essentially flow out.

In terms of energy use, PID won't get you anything. In fact, contrary to conventional wisdom and common practice, VFDs won't save you money on a small lift station. It takes a certain amount of power to lift a given volume of water to a specific height. The only things that you can play with are total head and pump efficiency. Most pumps operate at peak efficiency near the design point. Therefore, intermittent operation at full speed is going to be more efficient from a kWh viewpoint, because you don't loose any pump efficiency and you don't have VFD losses.

There are only two pump station applications where you come out ahead on energy cost with VFDs:

One is if the pump station has to pay demand charges, which is only true in larger pump installations. In that case you can get some cost savings from eliminating frequent starts at full pump load.

The second is if you have to maintain a constant flow or pressure rise through the pump station and you use a VFD instead of throttling the discharge with a valve. This situation is usually only the case in industrial applications. Both potable water pumping and sewage lift station pumping generally work just fine without VFDs and are just as energy efficient.

10 HP pumps have the VFDs. Analog level I receive from Miltronics. Analog flow – from Flow meter. The objective is: to prevent the wet well from overflow with energy efficiently and equipment life time maximizing.

We don’t need neither to maintain certain precise level nor flow. My boss has great water treatment experience and he believes that his algorithm (25% flow -> reduce speed to 60% …) save energy and increase pump lifetime. During year and a half I’ve been working for this company, we implement more then 10 water projects. Basically, PLC programming was provided by third party engineers. Most projects didn’t have speed control at all, just start/stop with 100% speed. Current project, finally I have convinced my boss to write program ourselves. He invented the speed control algorithm and allows me to write the program. Recently hired graduate probably influents boss fashionable word not really understanding the algorithm. I asked grad: “why do you want to use PID”. The answer: “It more reliable and will work smoother”.

Well. Let’s think about. What we can assign as PV? Let’s try Level. Pump has capacity several times higher then inlet flow. Not more then in 1 minute the pump emptied the wet well. It will easily to maintain the certain level by ramping all the time and wasting energy.

The discharge flow might be a good chose for energy saving purpose but it is nothing to do with our algorithm.

Only one value, as a PV, related to our algorithm is the motor speed. In this case I need more then one Set point. Fortunately Direct Logic provides Ramp/Soak Generator. I would generate speed Ramp of Set Points from 60% to 100%. No doubt this solution will work smoother. I’m not sure about reliability, may be. Even it would be easy to program, but not simpler for the operators to work with, as Tom said. Problem is we don’t have any feedback from motor speed as analog RPM.

So I still have no idea why I need PID in this particular case?

Thanks.

Two points:

First, if you're going to use PID, it's a good idea to make the SP, PV and CV all have the same units. That's because the PID algorithm calculates the difference between the SP and the PV (Error) and uses the result along with the tuning values to calculate a new CV.

In this case, the SP is the wet well level (Inches) the PV is flow (In^3/Min), and the CV is pump speed which is directly related to flow. Has anyone ever told you "you're comparing apples to oranges"? That's akin to what you're doing when you calculate an error by subtracting flow from height.

So, along with all of the other reasons offered, there's a mathematical reason for not applying PID to this system.

Second point:

You mentioned that the PID algorithm gives you a ramping function. So does any VFD worth its salt. You have control of the acceleration and deceleration rates in your VFD. Use them to ramp the speed changes under your current system.

Steve Bailey said:

Two points:

First, if you're going to use PID, it's a good idea to make the SP, PV and CV all have the same units.

That is what I am telling to my opponents.


You mentioned that the PID algorithm gives you a ramping function. So does any VFD worth its salt. You have control of the acceleration and deceleration rates in your VFD. Use them to ramp the speed changes under your current system.

Click to expand...

Yes certainly we use VFD acceleration function. In my post I mentioned about Ramp/Soak Generator. It means the PLC generates series of SPs instead of manually. I can analyze whether the Level still increasing or not. But I agree with you. We don’t have to overcomplicate control.

I think this is one of those applications where "proportional only" will work fine since the process has an integrator.

If the object is to balance flow in and flow out then a proportional-only loop controlling level with pump speed should do the job. The level represents the integral of the difference in flow rates. Chose a level setpoint that best suits the pump characteristics.

You'll probably want to limit the minimum speed for efficiency's sake. That implies drawing the level down at low inflows, so you would need to stop at some pre-determined low level and wait for a high level to re-start.

how many people here know that running centrifugal pump at full speed against a closed valve will draw less current that a centrifugal pump running half speed against an open valve??

Savings in shut off head = smoked seal

darrenj said:

how many people here know that running centrifugal pump at full speed against a closed valve will draw less current that a centrifugal pump running half speed against an open valve??

Click to expand...

Running a centrifugal (and others) pump at shut off head with NO FLOW thru pump does not cool the seals. Seals MUST stay lubricated thru leakage and FLOW - shutoff head can heat up the whole pump unit also. Sure it is less power, but the cost of seals after you burn them out eats up the savings many times over. MUCH better to use a bypass to ENSURE flow thru pump at all times THEN you can throttle.

AFFINITY LAW OF PUMPS AND BLOWERS
1. Flow proportional to speed
2 Head is the square of speed increase
3. Power is the cube of speed increase.

You can save on power by thottling pump or using bypass (preferable).

VFD will save much more than throttling because it is operating on a cube. This is widely written up in many journals.

Flow would be dealt with by adding pumps or securing them as flow rises or falls.

I would tend to start a VFD pump in this plant at slow. Lot easier on starter and couplings etc etc. THEN bring up the speed to match that of others -- maybe you would have to measure head on each pump to balance that out and ensure they are all at the same area of their head / RPM / flow curves. Maybe having all at same speed will be close enough.

I think you guys are on right track Alex. I think you have about same weather as we do in Seattle.
Flow would be dealt with by number of pumps on line and then pump speed. Of course this impacts level. I think the easiest least expensive and easiest to maintain is multiple pumps with several stages of (one switch for each pump -- rats that fouls up ease of keeping service time equal)

IF you go with PID I think I would tend to control level.

Not an easy issue you got flow, level, number of pumps on line, try to keep service time equal. Lessee how else can we complicate this??


Dan Bentler