Automatic Pump Spillback Flowmeter Design

1. Purpose

This procedure explains the type of primary element and its location for flow measurement in an automatic pump spillback system.

2. General

Automatic pump spillback requirement for pump protection (Type C) is determined by following procedure .

3. Automatic Spillback Design Policy

3.1 Background

Prior to October 1997, Inflection Point Engineering policy located an orifice plate in the pump discharge piping as the primary element for flow measurement in an automatic pump spillback system. The main drawback with this arrangement was two orifice plates in series (one in pump discharge, one in forward flow) leading to two long meter runs in the larger pipe sizes.

In October 1997 an alternate design was developed to avoid two orifice plates in series. This design incorporated a primary element (orifice plate) in the spillback line, and summed the spillback flow with the net forward flow to provide the process variable for the minimum flow spillback controller.

Operators, as well as clients’ engineers, during review meetings expressed uneasiness with the perceived “complicated” spillback system. Also, for systems which included a low flow shutdown associated with the pump, the original spillback system (orifice plate in the pump discharge piping) was used in order to avoid using the summed spillback and forward flow signal as the process variable trip. This sometimes led to two different styles of automatic pump spillback systems within the same project. Some clients requested the “old” style spillback design for all of the pumps that required a spillback. This ended up causing rework.

3.2 Current Spillback Design

Inflection Point Engineering has renewed its automatic pump spillback policy to standardize on locating the primary element in the pump discharge piping once again.

The primary element in the pump discharge piping will be an orifice plate for all pipe sizes.

Note: The pressure drop considerations for both the spillback orifice plate and the forward flow orifice plate must be accounted for in the hydraulics. See .