With the proliferation of dedicated digital anti-surge control systems over the last 20

years, there has been an associated proliferation of promises and myths about the

surge margin. In this post we'll try to explain the need and use of the surge margin.

What is surge margin?

The simplest definition of surge margin is the distance of the operating point from the

surge line. This definition requires some additional qualifiers in order to make it useful

as a comparison criterion for the control system performance. The compressor

performance map is commonly used for defining the performance of a compressor

section (or stage). The performance map is a useful device for relating the anti-surge

system performance to the compressor operation.

How is the margin calculated?

The surge margin is calculated by dividing the distance of operating point from the

surge line at the same pressure ratio by the total flow scale. The minimum control line is

based on the same technique. Therefore, if the minimum control line is set at 10%, when

the operating point is on the control line, the operating point is 10% away from surge line

(on a constant pressure ratio basis) as compared to the full range of the flow scale. If the surge

flow at this pressure ratio is 30%, for example, then the surge margin relative to the compressor

surge flow is 10% / 30% = 33%.

The standard technique used by Tri-Sen for establishing the minimum control line is to

use a constant offset. A proportional offset can be used as well that has the effect of

changing the surge margin as a function of the full scale of the flow term to a function of

the surge line flow. The proportional approach is appropriate for use on in applications

that have a widely varying molecular weight. Whether or not to use this technique

should be decided during the execution of the control projects.

These graphics show the two methods  available for determining minimum surge  margin.

How much margin is appropriate?

Regardless of the technique used to measure the margin (constant margin or

proportional margin), the amount of margin required for a particular installation depends

on a number of factors. The amount of margin required to protect the compressor is not

necessarily the same as the amount of margin required to protect the process. A simple

analogy is highway traffic. How closely can you follow the car in front? The more

distance you leave between you and the car in front, the more time you have to react

and the less abruptly you will need to react. But in heavy traffic, it you leave too much

room, other cars will cut in front of you. You have to make the decision based on

several factors.

In a compressor installation, the most significant parameter that will affect the

appropriate margin is the sizing and reaction time of the anti-surge valve. If the valve is

inadequately sized or opens too slowly, a wider margin will be required to prevent

surge. Even if the valve is adequately sized, the impact on the process needs to be

considered. If the minimum margin is small, the valve will have to open more abruptly to

prevent surge, upsetting the process.

For processes that routinely operate near the minimum control line, a wider margin will

tend to waste power. The process dynamics as dictated by piping lengths, valve

locations, vessel and piping volumes and mechanical equipment is a big influence on

the required margin that must be considered.

How is the minimum margin determined?

Traditionally, trial and error was the most common technique for finding the required

margin. For installations that routinely operate far away from surge, a conservative

margin is chosen (10% of the full flow scale). The only time the anti-surge controller is

active is during startups, shutdowns and significant load drops. But compressors that

routinely operate near the minimum control line will benefit from a tighter margin, if the

system can tolerate it. In the past, about the only way to evaluate the minimum margin

requirement was to test the control system on the live process. Over time, if the

protection proved to be inadequate or if the process disruptions were too severe, the

margin would be increased.

In the last few years, the availability of rigorous, high fidelity dynamic simulation

software  has allowed anti-surge control system designers to evaluate the dynamics

of the system and test the control system performance before the control system is

even installed. This software can be connected directly to the project control system

software, allowing the actual project software to be tested rather than just a simulation

of the control software.

Other influences on margin selection

A good anti-surge control system does not rely just on the controller margin to protect

the compressor and the process. Other features that need to be included are

performance controller decoupling, adaptive tuning, load loss anticipation and open loop

response. And of course the experience and qualification of the engineering staff that

applies the control system is the most important feature that should be evaluated when

selecting a compressor control system supplier