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Graftel has become a
reseller and calibration source for Fox
Valve ! This means that we can
now bring you calibrated sonic nozzles and calibrated sonic chokes at a
very competitive price!. Contact a Graftel
Representative today to receive a quote on a calibrated sonic
choke! Fox valve also is a manufacturer of: a Graftel
Representative today to receive a quote on a calibrated sonic
choke! Fox valve also is a manufacturer of: a Graftel
Representative today to receive a quote on a calibrated sonic
choke! Fox valve also is a manufacturer of:
Critical flow nozzles, also called sonic chokes, have been used to maintain stable gas flow rates in systems where maintaining
stable, accurate gas flow rates is essential to overall system performance. By establishing a shock wave in the
venturi, the sonic choke establishes fixed flow rates unaffected by P or any fluctuations, surges, or changes in downstream pressure.
With no moving parts, sonic chokes provide the simplest and most reliable way to regulate gas flows. This is particularly true when high pressure, high
temperature, cryogenic, explosive, or high purity gasses need to be regulated.
Industry uses many different names to describe this equipment. Here is a partial list of descriptions that can be used interchangeably:
Sonic chokes are converging/diverging nozzles whose operating principles are discussed in every fluid mechanics textbook. They offer the designer a
remarkably simple way to regulate stable gas flow rates.
Simplifying Gas Flow Control by Eliminating Flow Meters, Feedback, and Control Valves
Systems designers often think that the only way to maintain uniform gas flow rates into a process where backpressure may fluctuate is to provide for
continuous flow management via a) a flowmeter, which sends a signal to : b) a process controller, which in turn adjusts c) a control valve. The
aerospace industry, which cannot afford the weight and complications of such a system, embraced critical flow venturies thirty years ago as the
simplest and most reliable way to regulate gas flow rates. As long as inlet pressure to the sonic chokes can be regulated accurately, all of the above
equipment can be replaced with one simple sonic choke. A simple, standard piece of equipment - a pressure regulator has, when coupled with a sonic
choke, become a gas flow regulator.
How does Pressure Drop Affect Flow?
It doesn't. As long as discharge pressure is below about 88% of upstream pressure (in psia), backpressure has absolutely no effect on flow rate.
Downstream pressure can fluctuate wildly with no impact on gas flow rates. This limitation is referred to as the 'recovery' of a choke.
Recovery can be assumed to be 88- 90%. For example, with a fixed inlet pressure of 100 psia, a sonic choke can discharge into a
backpressure that cycles abruptly from 20 to 80 to 20 psig, and will deliver stable, uniform, accurate, unchanging gas flow rates.
Pressure/Temperature Ratings
Because they are machined from solid bar, sonic chokes are often used in gas lines at 3000 psig or higher. With no moving parts, they are the ideal
way to regulate gas flows that are very hot, or very cold. Sonic chokes have been used with gasses up to 1500° F, and with Helium at - 400° F.
Calibration
Sonic chokes are delivered with a standard, theoretical calibration of ±2%. However, our chokes can be provided with fully NIST-traceable
calibrations accurate to ±1/4%. Add an additional 4 weeks.
Accuracy
Sonic chokes can be calibrated to ±0.25% traceable to NBS. Theoretical calibrations are accurate to ±2%. There is no reason to ever install a
flowmeter downstream of a sonic choke. The flow has been regulated with such accuracy that it does not need to be measured. Simple corrections
enable calibration data to be converted to other temperatures and pressures.
Controlling Conveying Air Flow Rate in Dense Phase Pneumatic Transport Systems
Fox sonic chokes have been incorporated into hundreds of dense phase pneumatic conveying systems. In these systems, it is essential that air flow
rates are both controlled accurately and remain fixed and stable, even if downstream pressures due to changing product or flow rates (sugar, sand,
coal, etc.) vary. The sonic choke must, therefore, maintain uniform air feed rates, and hence uniform, controlled transport velocities, that are unaffected by any changes in product rates or characteristics.
Ultra-High Purity Gas Systems
Traditional flow meters used by the industrial gas industry would contaminate the ultra high purity gasses needed by the semiconductor and other
industries. When filling customer tanks without a sonic choke, gas feed rate vary constantly as P varies. With a sonic choke, which are easily
provided in ultra-high purity configurations, flow rates are easily controlled and the lack of moving parts eliminates the chance of contamination.
Rocket Engines, Chemical Lasers, and Superconductors
Fox sonic chokes have been used in these three applications for decades. Aerospace - Chokes are typically used to control hydrogen and oxygen, the
fuel and oxidizer, in test-stand rocket engine firings as well as flight hardware. Pressure ratings are frequently 4000 psig or higher. Chemical Lasers
similarly require precise flow control of very high pressure gasses. Airborne lasers require light weight flow controls, for which sonic chokes are ideal.
Superconductivity research often requires controlling helium or hydrogen gas flow rates at below -300° F, which is quite simple for chokes.
Calibration Standard
Because sonic chokes are so simple, they have been preferred for years as a reference standard. One calibrated choke can be used as a reference
to which other, less reliable flowmeters can be checked. All critical flow nozzles are custom-built to meet your unique flow control requirements. Typical delivery is about 5 - 6 weeks. This can sometimes
be expedited with a rush premium.
Material of Construction
Chokes can be specified in any machineable material. Standard materials are 304 and 316 stainless, brass, and carbon steel. Fox can, and does,
build them in many other materials, including titanium, monel, zirconium, Alloy 20, bronze, and others.
End Connections
Two of our most standard installation geometries are shown at right:
Flange-insert style (for 1" - 6" lines)
Tube-insert style (for tube sizes below 1") However, any other end connection of installation geometry can be specified, including any type of flanges; NPT, BSP, or metric threads; butt-weld or
socket-weld ends; VCR or sanitary connections.
Pressure/Temperature Rating
Fox chokes are machined from bar and can be designed for service at 8000 psig or higher. Because they have no moving parts, they are ideal for use
with cold or hot gasses between -400° F and +1500° F.
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1. Why should I care if there is a shock wave at the throat of the venturi?
Because once that shock is established, nothing that happens downstream can influence the flow pattern going into the venturi. (No downstream
perturbations can propagate upstream). That means that once you have locked the venturi into one flow rate, it is completely unaffected by anything
downstream, unlike P devices. Flow rate is now fixed, stable, and repeatable and is solely a function of upstream pressure, not P!
2. Downstream pressure fluctuations have absolutely no effect on flow?
Sorry. As long as downstream pressures stay below 88% of upstream pressure, they can fluctuate wildly, while flow rates remain constant, stable,
repeatable. This is what is meant by Recovery - the maximum ratio of downstream to upstream pressures (in psia) where flow through the nozzle
remains choked.
3. So critical flow venturies convert a pressure regulator into a flow regulator?
Correct. Once you know the characteristic curve of the nozzle, you can crank in any flow you want simply by setting the inlet pressure to the nozzle.
Downstream conditions are irrelevant.
4. How accurately can they control flows?
We can guarantee theoretical flow rates through our nozzles to ±2% , although overall system accuracy will depend upon your pressure controls and
instrumentation. NBS-traceable calibrations are available and are accurate to ±1/2%.
5. But I thought I really wanted to meter flow?
Many of our callers are looking to meter flow so they can use a control valve to set, or control, a predetermined flow rate. They really want to regulate
flow, and use a control valve with a flowmeter to do so. How about simply regulating flow rate with such high accuracy that it doesn't need to be
metered?
6. Why don't we hear more about them in the control and instrumentation publications?
Chokes have been around since Bernoulli so they are not new. And they're inexpensive. You don't need a lot of feedback and controls to generate a
stable flow rate. Its as simple as you can get. Maybe that's the reason.
7. Are they available from stock?
No. Every sonic choke is built to order, with a delivery of about 4 - 6 weeks. Standard materials are stainless, brass, monel, and titanium. Standard
styles include flange insert, tube insert, flanged, butt weld, and socket weld.
Contact
a Graftel
Representative today to learn more about purchasing a Calibrated Sonic
Choke or Sonic Nozzle.
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