Archive: Mar 2018

Applications of the MAC STM and STMT Steam Flow Meters and Transmitters

Steam flow rate is typically measured using orifice plates, nozzles, venturies, or vortex sheading flow meters. All of these devices respond to volumetric flow, not true mass flow. For any fixed volumetric flow, the mass flow will vary with the line pressure. If the absolute pressure is doubled, the density of the steam will double and the mass flow will double even though the volumetric flow rate displayed by the metering device does not change.

In many applications the nominal line pressure is known and assumed to be relatively constant. In this case the flow metering device is calibrated at the nominal line pressure and any mass flow error due to line pressure change is tolerated.

Most steam systems are operated above nominal pressure at low flow conditions. As steam usage increases the boiler pressure often falls below nominal pressure. At locations far from the boiler it is not uncommon to see the line pressure fall to one half the nominal pressure under maximum flow conditions.

The graph in fig. 1 shows the error in mass flow rate caused by line pressure deviation from nominal conditions for conventional flow rate meters.

In applications requiring high accuracy, the line pressure can be measured and used to correct the volumetric flow measurement to obtain true mass flow. Flow measurement systems that do this automatically are available. They are quite complex and very expensive.

In some cases a pressure regulator is installed immediately upstream of the flow measuring device in order to maintain a constant line pressure regardless of the flow rate or boiler pressure. Again this complicates the system and increases the initial cost as well as the cost of maintaining the system. The MAC true mass flow rate steam meters and transmitters operate on a unique principle which allows accurate mass flow rate indication which is unaffected by changes in the line pressure.

Principle of Operation

The MAC STM Series Steam Meters use a critical flow nozzle and a single pressure gauge to indicate the true mass flow.

The critical flow nozzle allows the steam to reach sonic velocity at the throat of the nozzle at a very low upstream pressure. Due to the laws of physics the sonic velocity at the throat cannot be exceeded. As the upstream pressure increases the volumetric flow rate does not change, but the density of the steam increases with increasing pressure and thus the mass flow rate is proportional to the upstream pressure. The standard MAC Steam Flow Meters indicate 100% flow when the inlet pressure is 75 PSI. A conical diffuser reduces the sonic velocity at the throat back down to the flow velocity in the pipe while recovering up to 90% of the upstream pressure.

Because of the pressure loss across the nozzle this type of meter can only be used in applications where steam is injected into a lower pressure area. Typical applications include humidification, sparging tanks, atmospheric blanchers, steam eductors, steam injection into process ovens, and any other application where steam is exhausted through a manifold or nozzles to a lower pressure.

There are many applications like those mentioned above where the MAC Steam Meters and Transmitters provide a simple, low cost, and accurate way of measuring steam mass flow rate.

Application Comparison

Fig. 2 shows a typical arrangement for measuring and throttling the flow of steam into an injection process. Let us assume that pressure P4 is atmospheric pressure and that P3 must be 40 PSI above P4 to get to 100% of the desired flow through the manifold and spray nozzles. Also assume that a delta P of 5 PSI across the orifice is equal to 100% flow. When the throttling valve is closed and there is no steam flowing, P3 is equal to P4, and P1 is equal to P2.

When the flow rate is 100%, P1 will drop to 90 PSI, P2 will be 85 PSI, P3 will be 40 PSI. The pressure drop across the throttling valve is equal to P2-P3 which will be 45 PSI.

When a MAC Steam Flow Meter is used, it must be down stream of the throttling valve. This is because with the valve closed, the Mac Steam Flow Meter would indicate full line pressure and full flow if it was upstream from the valve.

Fig. 3 shows the same application with a MAC Steam Flow Meter. The throttling valve has been placed upstream of the MAC Flow Meter. When the valve is closed and there is no steam flowing, P2, P3, and P4 are all at atmospheric pressure. When the flow rate is 100%, P3 will again be 40 PSI. The MAC Flow Meter will indicate 100% flow rate when P2 is 75 PSI. The pressure drop across the throttling valve is then P1-P2 or 15 PSI.

The flow meter in Fig. 2 will indicate approximately 10% low at 100% flow rate due to the drop in line pressure. The flow rate indicated by the MAC Flow Meter in Fig. 3 will be correct even though the line pressure falls to 90 PSI as the valve is opened and the flow rate increases.

Athough the MAC Flow Meter requires 75 PSI upstream to get to 100% flow, up to 90% of the absolute pressure is recovered and is available to cause flow through down stream restrictions such as pipes, Manifolds, and spray nozzles.

MAC Steam Flow Rate Transmitters operate on the same principle as the MAC Steam Flow Meters. The transmitters include a gauge for local flow indication and a pressure transducer and interface electronics to generate an electrical output of zero to 5v DC or 4-20 mA.

What’s Wrong with Relative Humidity Above 100°C?

Water vapor above a temperature of 100°C at atmospheric pressure is in the super heated state. The characteristics of the relative humidity scale change drastically in the super heat area. It is somewhat like going through the sound barrier, the physics take on new characteristics in the super heat area just as they do in the super sonic area. This can be confusing if not prepared for the change.

Below 100°C it is possible to achieve 100% relative humidity at any temperature. Above 100°C (in the super heat area) the maximum possible relative humidity plunges rapidly as the temperature increases. Pure steam (100°C dew point) will register only 20% on the relative humidity scale at 150°C. At 175°C the maximum possible RH is only 10%. At 200°C, maximum RH is only 5.9% and at 370°C, maximum RH is a mere .48%.

An RH instrument with a published accuracy of +/-1% will only indicate true moisture level with an accuracy of +/-5% at 150°C (+/-1% out of 20%), +/-10% at 175°C, +/-17% at 200°C, and will be useless at 370°C, even if the sensor is not destroyed by the high temperature.

Most RH instruments have a reduced accuracy specification above some specific temperature. An instrument with a published accuracy of +/-1% may be +/-2% above 90°C. This is sometimes hard to find on the data sheet, but is usually there somewhere. If the RH accuracy drops to +/-2% at elevated temperatures the true moisture level will have an accuracy of +/-10% at 150°C, +/-20% at 175°C, and +/-34% at 200°C.

Even if we forget about the absolute accuracy of an RH instrument above 100°C, the RH scale itself has problems in the super heat area. An RH instrument that is totally accurate may indicate 10% RH at a temperature of 150°C. This point is equivalent to 50% moisture by volume, or a dew point of 82°C or a humidity ratio (lb water vapor/lb dry air) of .622. This same instrument will display 10% RH at 175°C at 100% moisture by volume, or a dew point of 100°C, or a humidity ratio of infinity.

The above shows that the relative humidity scale is essentially useless or even misleading at temperatures above 100°C.

Some relative humidity instruments can indicate dew point or another absolute humidity scale. This is done by measuring RH and temperature and then calculating absolute humidity. This method is not accurate above 100°C because of the limited maximum indication on the RH scale.

For more in depth information on this subject refer to pages 7 & 8 of the
“MAC Humidity/Moisture Handbook.”

The Mac Humidity / Moisture handbook can be viewed online.

What’s Wrong with Relative Humidity Above 212°F?

Water vapor above a temperature of 212°F at atmospheric pressure is in the super heated state. The characteristics of the relative humidity scale change drastically in the super heat area. It is somewhat like going through the sound barrier, the physics take on new characteristics in the super heat area just as they do in the super sonic area. This can be confusing if not prepared for the change.

Below 212°F it is possible to achieve 100% relative humidity at any temperature. Above 212°F (in the super heat area) the maximum possible relative humidity plunges rapidly as the temperature increases. Pure steam (212°F dew point) will register only 20% on the relative humidity scale at 300°F. At 350°F the maximum possible RH is only 10%. At 400°F, maximum RH is only 5.9% and at 700° F, maximum RH is a mere .48%.

An RH instrument with a published accuracy of +/-1% will only indicate true moisture level with an accuracy of +/-5% at 300°F (+/-1% out of 20%), +/-10% at 350°F, +/-17% at 400°F, and will be useless at 700°F, even if the sensor is not destroyed by the high temperature.

Most RH instruments have a reduced accuracy specification above some specific temperature. An instrument with a published accuracy of +/-1% may be +/-2% above 200°F. This is sometimes hard to find on the data sheet, but is usually there somewhere. If the RH accuracy drops to +/-2% at elevated temperatures the true moisture level will have an accuracy of +/-10% at 300°F, +/-20% at 350°F, and +/-34% at 400°F.

Even if we forget about the absolute accuracy of an RH instrument above 212°F, the RH scale itself has problems in the super heat area. An RH instrument that is totally accurate may indicate 10% RH at a temperature of 300°F. This point is equivalent to 50% moisture by volume, or a dew point of 179°F or a humidity ratio (lb water vapor/lb dry air) of .622. This same instrument will display 10% RH at 350°F at 100% moisture by volume, or a dew point of 212°F, or a humidity ratio of infinity.

The above shows that the relative humidity scale is essentially useless or even misleading at temperatures above 212°F.

Some relative humidity instruments can indicate dew point or another absolute humidity scale. This is done by measuring RH and temperature and then calculating absolute humidity. This method is not accurate above 212°F because of the limited maximum indication on the RH scale.

For more in depth information on this subject refer to pages 7 & 8 of the
“MAC Humidity/Moisture Handbook.”

The Mac Humidity / Moisture handbook can be viewed online.

Accurate and Responsive Humidity Probes by MAC Instruments

Since 1990, MAC Instruments have been manufacturing industrial moisture measurement instruments. With a MAC Instruments moisture analyzer one can instantly measure absolute humidity in the most demanding high-temperature conditions. MAC’s moisture analyzer range renders all the necessary data through a simple, clear interface. All our products are used in several high-temperature humidity measurement applications at different installations across the world.

The Efficient Features of MAC 155 Humidity Probe

MAC155 is implemented in worldwide industrial operations for 24/7 monitoring of stack emission gases. It is designed and built for measuring humidity found in exhaust gases in temperature ranges up to 1200°F. Our MAC155 moisture/humidity probe is core-compliant with US EPA regulations. This device strengthens your process safety and makes it user-friendly as it consists of several high-end components like humidity transmitter and probe. Moreover, it is also protected by a sintered filter element made up of a 10 micron ceramic element that can be conveniently cleaned and replaced without any hassle.

MAC155 has plenty of features and characteristics. You can find more information on our website.

Furthermore, we also provide a range of special accessories to extend the use of our probes in temperature ranges up to 2400°F. All the probes we offer are exceedingly sensitive to water humidity/vapor. They do not depend on any optics, chemicals, wet bulb, mirrors, or compressed air to calculate readings. All our products are highly responsive to the partial pressure of water humidity/vapor enabling instantaneous readings. Please visit us at www.macinstruments.com for any further concerns or assistance. Also drop us an email at moc.stnemurtsnicam@ofni or speak with our MAC representative at 419.621.2322.

MAC Instruments Offers Industrial Compliant Moisture Measurement Equipments

MAC Instruments is the leading manufacturer of steam flow meters, transmitters, and industrial moisture analyzers in the industry. We mainly manufacture these high-end products to serve high-temperature industrial applications. Our entire range of analyzers provides all the data you need through a simple and clear interface. Our moisture measurement instruments offer NIST-traceable direct moisture level measurement that helps you meet with emission monitoring needs.

Moreover, MAC moisture analyzersintroduce high performance that helps you to take an advanced scientific and result-driven approach towards understanding humidity levels in your industrial process.Since 1990, we have been in this industry, and are producing moisture measurement devices to serve every industrial sector with our moisture controlling products. All of our devices easily enable you to immediately measure absolute humidity in the most unfavorable conditions.

All MAC moisture measurement instruments and products that we offer are unlike any other products available on the market. And the aspects that distinguishus from the rest are the techniques and methods we use in order to manufacture moisture control equipments. Wedo notuse chemicals, optics, or compressed air to measure moisture, or any wet bulb techniques. Alternately, MAC products integrate a proprietary sensor that offers consistent accurate readings and performance even during adverse climatic conditions or highly variable humidity conditions. MAC Instruments uses high-quality materials in all of our products to ensure that our customers receive error-free results on every use.

The presence of moisture is inevitable in our surroundings, and this particular aspect can build immense negative impact on various process efficiencies. In that case, MAC Instruments has emerged as the most trusted source for moisture control activities.

You may drop us a line at moc.stnemurtsnicam@ofni or simply visit our website www.macinstruments.com for more detailed information. Also, if you want to directly get in touch with our MAC representative, call us at 419.621.2322.