Solving Push-Through Flow in Positive Displacement Pumps
Application: Downstream – Buffer Dilution
Media: Ambient WFI, Buffer, Ultrafiltrate, Diafiltrate
Valve Requirement: Pressure control upstream or downstream to eliminate push-through flow
The Problem: Push-Through Flow
Elevated buffer head pressure can cause push-through flow on positive displacement (PD) pumps. When head pressure is high enough, it defeats pump suction line check valves, allowing flow through the pump when it stops—and in some cases, adding uncontrolled flow even when the pump is running.
This presents significant issues, particularly at low flow set points. The pump is unable to control flow below the rate of the push-through, which decreases the practical control range of the pump and reduces its accuracy.
How Push-Through Flow Affects Dilution Accuracy
Push-through flow directly and negatively affects dilution accuracy, increasing the time it takes to produce in-spec diluted buffer. Non-spec (improperly diluted) buffer must be diverted to drain—it is only directed to the chromatography column when it meets specification.
The result is twofold: batch time delays and loss of expensive buffer. The same issue applies to formulation applications, where push-through flow can affect accuracy when delivering to a collection vessel rather than a column.
Traditional Approach: PRV or PCV Upstream of the Pump
The traditional method for reducing elevated buffer head pressure is to install a pressure relief valve (PRV) or pressure control valve (PCV) at grade before the pump inlet. There are two common configurations:
Configuration 1: The PRV is installed upstream of both the pump inlet and the buffer distribution block valve. Steriflow MK96 PRVs (1/2” and 3/4”) are commonly used in this position on inline dilution (ILD) skids.
Configuration 2: The PRV is installed immediately before the pump, downstream of the buffer distribution block valve. Steriflow MK96 PRVs (1-1/2”) are used in this position to reduce buffer head pressure directly upstream of PD pumps.
Limitations of Traditional Valves in This Application
Regardless of where the traditional PRV or PCV is installed upstream of the pump, this is a challenging service for conventional valve technology. Key limitations include:
- Flow turndown limitations
- Low pressure set point limitations on PRVs
- Long-term damage risk from suction line pulsation
- Flashing and cavitation
Traditional PRVs and PCVs are used in this application, but it is important to understand their limitations and to provide complete application information to the engineers sizing your valves.
Note: A downstream pressure of 0 psig is not achievable with the MK96. The lowest usable set point is 2 psi. If the MK96 has no set point configured, the valve will operate at full Cv until 2 psi is reached, which can result in significant uncontrolled flow.
A Better Alternative: The Equilibar Back Pressure Regulator
There is an effective alternative to traditional PRV/PCV pressure control for eliminating push-through flow. Installing an Equilibar back pressure regulator (BPRV) downstream of the buffer pump and upstream of the pump outlet instrumentation can eliminate PD pump push-through flow entirely.
By adjusting the Equilibar set point pressure slightly above the maximum possible buffer head pressure, push-through flow is effectively eliminated.
Additional benefits of the Equilibar BPRV include:
- Stabilized pump head pressure – Eliminates the pump RPM/flow variability that can occur as pump head pressure changes
- Extended usable flow range – Particularly at low flows, where push-through flow can exceed the pump’s flow capability
- PD pump pulsation dampening – Reduces pulsation by approximately 25%
- Full pipe assurance – Ensures a full pipe to improve the accuracy of downstream instrumentation
Equilibar BPRV Product Options
Equilibar offers two back pressure regulating valve models suited for this application:
Equilibar Model FD – Multi-Use BPRV
The Model FD is a durable, multi-use back pressure regulating valve designed for repeated service in demanding bioprocess environments
Equilibar Model SD – Single-Use BPRV
The Model SD is a single-use back pressure regulating valve, ideal for single-use inline buffer dilution (ILD) skids. It is installed downstream of instrumentation to optimize pump head pressure and ensure a full pipe.
