Control Valve Flow Characteristics: Linear vs. Equal Percentage

Choosing a control valve by line size and a flow characteristic alone leaves important questions unanswered. Linear and equal-percentage trims describe how capacity changes with travel under defined conditions. A buyer also needs to understand how the valve will behave in the actual piping system. This guide explains the distinction and provides a practical way to compare quotations.

What linear and equal-percentage mean

An inherent characteristic is measured at constant pressure drop across the valve. A linear characteristic gives approximately equal changes in flow capacity for equal changes in travel. An equal-percentage characteristic gives approximately the same percentage change in capacity for each equal travel increment within its stated range. The latter therefore produces smaller absolute capacity changes at low travel and larger changes at high travel.

These definitions do not mean that 50% travel produces 50% of the maximum plant flow. The installed characteristic includes the effect of changing valve pressure drop and the surrounding system. Emerson's Control Valve Handbook explains this inherent-versus-installed distinction.

Build the RFQ around operating cases

Provide minimum, normal and maximum flow as separate cases, with upstream pressure, downstream pressure and temperature for each case. State whether pressure is gauge or absolute and whether flow is mass flow, actual volume or standard volume. Identify the fluid and its phase; for liquids include relevant density and vapor-pressure data, and for gases identify the composition and reference conditions.

Ask the supplier to return calculated capacity, selected rated Cv or Kv, predicted travel and the assumptions used for each case. Request the proposed trim characteristic explicitly. A quotation that states only a rated capacity does not show whether the normal duty and the lowest continuous flow fall within a usable control range.

Review more than the characteristic name

Equal-percentage trim can be a candidate when valve pressure drop changes substantially with flow, but it is not an automatic selection rule. Review the process objective, pump or compressor behavior, piping losses and controller strategy together. Ask what changes if downstream pressure falls or a parallel user starts operating.

Check the actuator, positioner and available air supply alongside the trim. Seat leakage, noise, cavitation, flashing and choked flow require their own assessment. A characteristic label does not establish suitability for these conditions. If digital signal characterization is proposed, document both the mechanical trim characteristic and the configured input-to-travel relationship.

A clear quotation comparison

For each offer, record the valve and trim size, rated capacity, characteristic, operating-case travel, actuator selection and any stated exclusions. Resolve differences before treating two offers as technically equivalent. For steam service, read our high-pressure steam control valve sizing guide before finalizing the operating data.

Send your duty cases and project requirements to WENZHOU ZHONGYUE VALVE TECHNOLOGY CO., LTD. for a project-specific discussion. Final selection should follow the agreed calculation, datasheet and purchase-order requirements.

Zhongyue High Quality Valve Manufacturer

The products are exported to the United States, Europe, the Middle East, Southeast Asia, Russia and other countries and regions, reliable quality and reasonable price. Reliable quality and reasonable price are the basis of our continuous development.

Contact

Get in touch Please leave the questions you need to consult, later we will have a professional technical team to contact you.

Email: sales@hizhongyue.com

Tel: +86 0577-28828932

WhatsApp/WeChat: +86 15658617888

© 2024. zhongyue valve llc.All rights reserved.

WhatsApp

WeChat