Comment

01 March 2003

No fire in the hole

By Patrick Lowery

Desirable alternative to classic electrical servo control loop for all clean gas and liquid delivery applications.

Grab a concept and make a splash.

Traditional pressure-based mass and volumetric flowmeters/controllers calculate a flow rate by measuring the upstream pressure, pressure differential, and fluid temperature across some type of flow restriction.

A different concept of pressure-based mass flow measurement control is on the way, and it employs single absolute pressure and temperature measurement to control liquids and gases with constant outlet process pressure.

It's an intrinsically safe, all-mechanical, volumetric flow producer that maintains the fluid control while sensors in the device acquire temperature as well as absolute and differential pressure for mass flow calculation.

The device maintains less than 0.5% stability for output and repeatability, 100-millisecond time responses, and less than 0.004% change in flow per 1-pound-per-square-inch change in inlet pressure.

Signal to a prime mover

Flow controllers operate in almost all industries that require the accurate delivery of fluids, either gaseous or liquid, to a chemical or mass transport process.

These controllers manage the rate of mass or volumetric flow into a process by utilizing a flowmeter coupled with some sort of proportional control valve. The controllers differ by their respective flow metering technologies.

Flowmeter technologies can be categorized into variable area, thermal mass, Coriolis, differential pressure, turbine, and oval gear. In all cases, regardless of metering technology, the flow controller is a classic servo control loop with the flowmeter providing the respective feedback signal.

The servo loop is the control of a single control element, such as the proportional valve, by means of the deviation in value of input—provided by the flowmeter and scaling circuitry.

• The flow sensor must provide an electrical signal to conditioning circuitry that provides linearization and amplification to the output signal.
• The amplified and conditioned sensor output is scaled according to different parameters such as zero offset, maximum scale, and fluid specific calibration multipliers to obtain a calibrated y = mx + b relationship.
• The scaled and calibrated flow sensor output compares with the user's desired set point or output. If the sensor output differs from the set point, the control circuitry will send an electrical signal to a prime mover, such as the valve, that opens or closes proportionally according to a predetermined transfer function.

This novel fluid control technology does not fit the scheme of a classic flow control servo loop. The flow sensor in this device only sets the position of the prime mover.

Once set, the controller does not need to constantly slew through the entire servo loop to maintain a constant volumetric flow output. For mass flow, the sensor will be used to move the prime mover in case of a deviation in gas density from that of the initial calibration conditions.

In essence, once set at the desired flow, the device will maintain flow through purely intrinsic or mechanical means, without the need for additional pressure conditioning devices.

Here are the salient concepts of this flow sensing and controlling technology as well as its advantages and limitations.

Thin film piezorestrictive

This fluid control device is an all-mechanical, volumetric flow–producing device. By maintaining a constant differential pressure across an adjustable flow restriction, the volumetric flow remains constant

The adjustable portion of the restriction assembly allows fine-tuning of the device to a specific value. The majority of the pressure restriction results from the effect of a laminar flow element (LFE).

The LFE reduces any turbulent or transitional flow—Reynolds number = N_Re > 2,300—to something smaller: N_Re < 2,300. This creates a linear relationship between differential pressure and flow rate.

Due to small inconsistencies in geometry or LFE permeability, it is very difficult to manufacture LFEs or porous restrictions with a precise pressure drop vs. flow characteristic.

Therefore, the adjustable secondary restriction upstream of the primary restriction allows for custom-tailored pressure drops with given flow rates in a manufacturing environment.

The pressure sensors used in the metering system are both thin film piezorestrictive strain gauges. They have a typical linear accuracy/hysteresis of ±0.1% full scale and a temperature coefficient of less than ±0.02%/°C. The temperature measurement device used is an integrated circuit temperature transducer with a temperature coefficient of 1 µA/K and an accuracy of 2.5 ± 0.8 K.

The total measurement uncertainty for the meter will be directly dependent on the accuracy of the pressure and temperature transducers, as well as the accuracy of the flow standard used to ascertain the flow controller's calibration constant.

The Darcy Equation describes the volumetric and mass flow relationships across a laminar flow or porous restriction that this technology leverages.

	(volumetric flow)
	(mass flow for gas)

where

d	=	hydraulic diameter or flow passage diameter
Pi	=	pressure upstream of restriction
Po	=	pressure downstream of restriction
P	=	pressure differential across restriction
R	=	universal gas constant
T	=	gas temperature
MW	=	molecular weight of the gas
L	=	length over which the pressure drop occurs
	=	fluid absolute viscosity
	=	material permeability (for porous media)
Z(P,T)	=	nonideal gas compressibility (function of pressure and temp.)

Purely mechanical means

The controller exhibits a linear relationship with pressure drop across the LFE, temperature changes, and changes in gas type, which allows a user to calibrate the controller with high precision.

In addition, the linear slope of the mass flow curves allows one to generate a mathematical relationship that will be very similar to the standard Darcy Equation.

Once corroborated with experimental data, the controller can calibrate for use with any hazardous gas and for use with a reasonable accuracy using only inert gases in production.

In addition, since the pressure differential across the LFE is constant regardless of various system conditions, a single absolute pressure measurement suffices for constant density fluids, such as liquids, and for gases venting to a constant outlet pressure.

The main differentiating feature of the controller in this study is its ability to maintain constant volumetric flow output by purely mechanical means. This allows the user to incorporate intrinsically safe flow control into hazardous or explosive environments.

Most, if not all, current flow controlling devices use solenoid-type proportioning valves as the prime mover. These devices require significant power and, by definition, do not meet the classification of intrinsically safe.

Here are some other advantages, in comparison to classic flow control devices:

• This device provides flow control; calibrated mass; or volumetric output, pressure regulation, and measurement in one single footprint. Almost all other flow control devices need a stable, regulated pressure input to function properly, due to pressure limitations in their valves.
• No need for constant power to a prime mover when using a stepper motor/encoder as a prime mover. Once the position sets, the device needs no further attention, unless there is a change in gas absolute pressure or temperature.
• The maximum flow range or span is scaled mechanically; therefore, it is impervious to drift or manipulation by means of electrical adjustments such as potentiometers.
• The mechanical device has a near infinite resolution as opposed to finite or discrete resolution, such as a digital thermal mass flow controller.

FMC controller flow control loop

Take away directive help

The mechanical volumetric flow device coupled with a positional and mass calculating feedback control loop provides a stable means of controlling the flow of fluids.

The device maintains flow outputs that are stable, repeatable, and linear with differential pressure, temperature, and absolute pressure. The volumetric output is also immune to variations in inlet and outlet pressure.

These performance specifications coupled with the inherent intrinsic safety of the device make this a desirable alternative to classic electrical servo-control loop technology for all clean gas and liquid delivery applications. FT

Behind the byline

Patrick Lowery is a registered professional engineer in California. He's the vice president of fluid technology and engineering at FlowMatrix Inc. in Carlsbad, Calif. Write him at plowery@flowmatrix.net.