This configuration is sometimes chosen to accommodate a specific plant layout. It is also used when we want to artificially increase pump head to bring it inside the operational envelope, a situation that occurs when we need to pump “across the aisle” (i.e., not upwards) from a pump box to a piece of equipment or tank located at the same elevation or slightly higher.
With this configuration, the question is: Which of the two static head values—SH1 or SH2—should be used as an input for a system calculation?
Discharging onto the Type 2 system happens as follows: When pumping is initiated, the liquid first reaches the high point elevation corresponding to SH1. Here it primes the siphon, which is shown in the clouded section of Fig. 2. Once flow is established and discharge from the end of the pipe is steady, the pump will deliver to the piping system with a static head of SH2. From this point on, the siphon section will work under a vacuum.
Based on the description above, it would seem that SH1 should be used for the calculations. However, while it may seem logical, this approach should be scrutinized for the following reasons:
A. The pump still needs to be able to develop a head that is sufficient enough to reach the highest piping elevation in order to charge the siphon. This is not assured if a fixed speed pump is sized with SH2. Looking at the typical pump curve, one can infer that, at a minimum, the elevation difference of DH (Fig.2) should not exceed the head gain between the established operating point head and the pump shutoff head on the corresponding pump curve DH1 (Fig. 3). Further, for practical reasons, the actual head gain should be less than DH1 to ensure that, during siphon priming, the pump flow isn’t too close to zero. Practically, it should be at least 15% or 20% of the design flow rate.