Foothills Gas Products Blog

Don’t let your pressure control get 'Droop-y'

Posted by Foothills Industrial Products on Jun 5, 2019 4:36:21 PM

Are you having problems with your fuel gas delivery system, burner train or the gas supply to your boiler?

What is 'Droop' and how is it caused?

An issue often referred to as ‘droop’ occurs when demand overwhelms the pressure control regulator. To be specific, when the immediate or continuous demand is near the regulator’s capacity, the regulator can become overwhelmed, causing outlet pressure to droop.

All fuel gas delivery systems, burner trains and boilers rely on having the correct pressure and flow delivered to meet the demand. Inefficiencies and problems occur at startup and with operation if this is not the case. The design and sizing of your regulator is critical to having the correct pressure and flow to meet demand. Find out more about the factors that can help minimize droop and ensure the success of your natural gas delivery system…

Factors to consider when designing a successful natural gas delivery system.

Droop is a characteristic of regulators when the flow rate is near its maximum level. A gas flow that experiences droop looks like this:

BlogImage-RegulatorDroop-Competitor

A balanced regulator design has a significant impact in minimizing droop.

The high demand can have an influence on the inlet pressure to the regulator (draw down of line pressure). The inlet pressure change can affect the ability of the regulator to control its outlet pressure delivery.

The term balanced simply means that both the downstream and upstream pressure of flow characteristics are taken into account, so that a change in the upstream pressure is immediately corrected by an adjustment in the downstream pressure outlet to ensure the set point is held. 

This adjustment can be instantaneous, and the ability of the regulator to react in an instant ‘is’ the expectation! The following image provides a great visual into how this works:

 

BlogImage-RegulatorDroop-BalancingMechanism-Trio

 

When selecting a regulator, attention must be paid to a) the capacity to meet the flow requirements and b) the accuracy throughout the full flow range. Instead of comparing flow profiles of one regulator vs. another, simply compare the Accuracy Class statements in the specifications, otherwise known as the AC spec. The AC specification indicates the bandwidth of accuracy of the regulator thru the full flow attributes, expressed as a percentage of the outlet set point. For example, a regulator with an AC5 specification, with a outlet setpoint of 2 psi, would not see the outlet pressure vary greater that 5% of 2 psi or 0.1 psi (1.9 psi- 2.1 psi).

BlogImage-PietroFiorentini-FE-Performance-1

Choosing the right product to the application.

Sounds simple but what does that mean? Lets look at the application flow profile to understand what the demand requirement is. A typical boiler application has an on/off flow profile, that is intermittent. The speed of response on startup and the ability to hold the set point during operation is critical.

 

BlogImage-RegulatorDroop-NorvalTest-2

 

Secondly, the materials that are used to achieve the pressure reduction are key.  In an on/off application, the turbulence that is repeatedly created drives substantial wear and tear on materials if they are rubber or soft in nature.  Typically, a stainless-steel plug, in a seat and plug style regulator is the best choice.

What is your application?

Click here to continue as we explore the right product for a few common applications.

Topics: Pietro Fiorentini, pressure control, natural gas, direct acting regulator, droop

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