Location of CW control valve

[isaivani]

Which would be the ideal position to put a control valve on the CW to a heat exchanger? Does it make a difference if the valve is at the inlet or outlet of the CW line.The heat exchanger in question is a carbon block type.
CW = (Cooling water)

 

[SeanB]

You should never put a control valve on the cooling water side of an exchanger. This is how you foul up exchangers. If you want to control temperature, control on the process side by bypassing a portion of the flow.

 

[isaivani]

What about a shut off valve instead of a control valve?

 

[SeanB]

If you are referring to using an isolation valve to control cooling water flow, as stated above you should not manipulate cooling water flow to control temperature. Many people do this but it is a no no. Cooling water tends to be dirty. As soon as you start reducing the cooling water flow rate in your exchanger you reduce the velocity and in turn foul up your enchanger much more quickly. If you want to increase the outlet temperature of your process fluid you should be bypassing on the process side and leave your cooling water rate as is.

 

[AndreChE]

It depends on the service of the heat exchanger but preferably, never in the CW side.

You may want to control the top pressure of a distillation column and have the possibility to change it, depending on the feed.

Our debutanizer has variable pressure and we control the pressure by a CW control valve in the condenser but it is the only case in an entire steam cracker plant.

However, fouling problems, depending on the water quality you have can be severe, mainly corrosion if the CW velocity are low.

AndreChE

 

[dcasto]

So how do I balance my water in a plant without a valve somewhere on the cooling water?. We always have a butterfly valve on the inlet and outlet. We adjust the outlet to balance our water.

 

[JRLAKE]

dcasto,

I have used VSD's on pumps feeding heat exchangers on both sides with success. It all depends on your system. Can you give more information on the system?

 

[dcasto]

With a plant that has several exchangers you'll need some way to make sure you get the proper split in flow. With a butterfly valve on the outlet, you Pinch all but one (or two) of the valves to get the desired flow through each exchanger. To set the flow, we had an orifice plate in the water line and we knew that at 20" water it was the right flow, so we would take a reading and manually adjust the flow to that point. We would move to the next exchanger and do the same. The last couple of exchangers(being the farthest away from the pump) we would not adjust. Sometimes the exchanger that was the one that needed the most control was in the middle of the header system. The goal is to have at least 1 or 2 exchangers without a pinched valve. What happens then, as your VSD slows down (because an exchanger is shut in?) it will lower the discharge pressure and the flow through all the remaining exchangers will be slightly larger than before (constant inlet pressure with a slightly lower outlet pressure across a partially open valve is more flow).

Now take this manual approach and add controls to the outlet butterfly and its 100% automated. In reality the should be a need to automate this but go ahead. If you really want to get exotic, measure the pressure drop of water across the exchanger and control that instead of the DP across an orifice.

 

[25362]

Throttling downstream may be needed when the HE unit is located at such an elevation that the return water pipe is in a vacuum; in which case dissolved air may be released impairing heat transfer.

 

[SeanB]

I think we are comparing apples to oranges here. Pinching a valve to balance cooling water flow is one thing. Controlling cooling water flow to manipulate the process side outlet temperature is a completely different animal.

 

[dcasto]

SeanB, you are correct. If you need an exact temperature out of a water cooled exchanger, then you balance the water flow for exchanger and than use a a hot side bypass with a temperature control loop on the mixed stream. I don't run across a lot of must be exact situations, so I didn't adress that situation.

 

[djack77494]

Balancing water flows through parallel paths is a common strategy, especially in older units where new equipment has been repeatedly added without any changes to the water system. This can work to a point, but you must recognize that you are decreasing the water flow through the older equipment each time you add a parallel path. I prefer using the lowest dP possible valves on the outlet sides of the exchangers if you go this route. Again, SeanB is right on.