When flow goes through parallel paths, both always have identical pressure drops. Flow rates adjust to balance the pressure drops. This often creates situations that don't meet process requirements. In such cases, plants usually install control valves to provide variable pressure drop in one or more parts of the system — to achieve satisfactory flow splits.
Figure 1 shows a typical case of split flow. Path 1 goes through a heat integration exchanger (E1). Path 2 goes through a utility exchanger (E2) to add incremental heat. The control valve on Path 1 (V1) alters the flow to meet the heat integration requirement of the other side of the service. The control valve (V2) on the utility heat adjusts flow to maintain a required downstream temperature.
The flow split between the two paths varies with the pressure drop on exchangers E1 and E2. If they differ in fouling tendencies or cleaning histories, the pressure drop through E2 quite possibly could be low enough that not enough flow would go through E1 even with V1 wide open. This means the heat integration step would remove insufficient heat.
Shifting the control valve position to V3 just may change the problem rather than solve it. With one valve in the V3 position, too much flow may go through E1 even if V3 is fully open. This occurs when E1 is relatively clean.
Depending upon the cleaning history and service requirements, the control pressure drop may need to be in either flow path. Keeping V1 and V2 in their original locations and adding a third valve at V3 provides the needed capability. Many different control configurations are possible. A common one relies on a split-range temperature controller on the E1 outlet to change both V1 and V3. The configuration shown uses a valve position controller. Process characteristics and objectives will determine the best choice among the different options.
Another alternative is to opt for a single three-way valve for controlling the process flow. Figure 2 illustrates two configurations. The first uses a three-way valve in splitting service upstream of the exchangers (V1a). The second puts the three-way valve in mixing service downstream of the exchangers (V1b). The better position will depend upon process characteristics including expected operating temperature, pressure and downstream disposition. The simple system shown combines the two flows. When downstream flows go to different destinations, the splitter configuration usually is used.
Buying and installing one three-way valve typically will cost less than putting in two separate single-flow valves. Nevertheless, plants often avoid three-way valves.
Historically, three-way valves generally were available with linear characteristics. So, systems needing equal-percentage or proportional characteristics weren't seen as good fits for the valves. Today, though, three-way valves come with linear, equal-percentage or proportional characteristics. They even can provide different characteristics for each path.
However, existing plant layout may work against using a three-way valve. If a single-flow valve already is in place, adding a second one often is cheaper than installing a three-way valve and doing the necessary piping reconfiguration.
In many systems, using a three-way valve creates a new common-mode failure case. Failure of the single three-way valve affects both E1 and E2. While failure of single-flow valves in parallel piping also causes interactions, the effects often are different. In some systems, the extra failure mode is relatively unimportant — for example, when a common failure mode already exists and contingency has been designed into the system. In other cases, addressing the new common-mode failure incurs extra expense to keep the plant safe.
Other requirements such as tight shutoff and special startup, shutdown or minimum flow requirements also might make a three-way-valve application more difficult.
Three-way valves are useful devices that deserve to be considered more often in process plants. However, you always should thoroughly check the possible implications of their use in an application.
ANDREW SLOLEY is a Chemical Processing contributing editor. You can e-mail him at ASloley@putman.net.