Three different process requirements
A plant may need to keep accepting liquid from an upstream operation, supply a receiver without long interruptions, or deliver a tightly controlled instantaneous flow into another process. These are three separate requirements. Alternating pressure vessels can solve the first two in a suitable design, but the third may require additional buffering or a different transfer method. Put the required measurement point in the specification.
What happens during changeover
The vessel finishing a transfer must stop gas admission and transition safely toward refill. The next vessel must already have sufficient product and permission to take over. Pressure build-up, valve travel and changing pipeline resistance can create variations during this transition. Product compressibility, gas entrainment and receiver pressure also affect the observed discharge pattern.
Translate the requirement into a test
For a simple storage duty, average flow over a defined interval may be the right acceptance measure. For a blending or dosing duty, the allowed deviation and time window may be much tighter. An illustrative specification might request a trend at one-second intervals during a ten-minute test, with the allowed variation agreed separately. The sampling interval is an example to discuss, not a universal requirement.
Decide where to provide a buffer
A properly designed receiver can decouple cyclic transfer from steady downstream withdrawal. Its useful volume must cover the longest credible interruption without reaching the low-level limit, and it needs suitable venting and overflow prevention. Buffer sizing should consider the real sequence, rather than multiplying an assumed flow by an arbitrary safety factor. Ask the supplier to show both the vessel cycle and the receiver level trend.
General engineering guidance. Numerical examples are illustrative; project specifications and acceptance criteria are agreed for the actual duty.