You would think that measurements of mass flow would be expressed in units of mass, such as grams/hour, milligrams/second etc. Most users, however, think and work in units of volume. That’s OK, at least when we are talking about the same reference conditions. Let me start with an example:
Mass versus Volume
Imagine you have a cylinder of 1 liter, which is closed by means of a moveable piston of negligible weight. This cylinder contains 1 liter of air at ambient pressure, approximately 1 bar. The weight of this volume of air at 0°C is 1.293 g, this is the mass.
When we move the piston half way to the bottom of the cylinder, then the contained volume of air is only ½ liter, the pressure is approximately 2 bar, but the mass hasn’t been changed, 1.293 g; nothing has been added, or left out.
According to this example, mass flow should actually be expressed in units of weight such as g/h and mg/s. Many users, however, think and work in units of volume. This not a problem, provided conditions under which the mass is converted to volume are agreed upon.
Using density in converting mass flow to volume flow
In order to use density in converting mass flow to volumetric flow, we must pick a set of specific pressure and temperature conditions at which we use the density value for the gas.
The conditions that are agreed upon contain various references, normal reference and standard reference, available in European or American style. What is de difference?
Normal reference, European style
Following the ‘European’ definition, a temperature of 0°C and a pressure of 1,013 bar are selected as ‘normal’ reference conditions, indicated by the underlying letter “n” in the unit of volume used (mln/min or m3n/h). The direct thermal mass flow measurement method is always based on these reference conditions unless otherwise requested.
An example conversion to volumetric units using Normal reference conditions:
The mass flow meter indicates 100 g/h of Air flow.
• Density Air (@ 0°C) = 1.293 kg/m3
• X ln/m Air = 100 g/h / (60 minutes x 1.293 kg/m3)
• Flow = 1.29 ln/m Air
Standard reference, European style
Alternatively, a temperature of 20°C and a pressure of 1,013 bar are used to refer to ‘standard’ reference conditions, indicated by the underlying letter ‘s’ in the unit of volume used (mls/min or m3s/h).
An example conversion to volumetric units using Standard conditions:
The mass flow meter indicates 100 g/h Air flow.
• Density Air (@ 20°C): 1.205 kg/m3
• X ls/m Air = 100 g/h / (60 minutes x 1.205 kg/m3)
• Flow = 1.38 ls/m Air
If the prefix ‘s’ has been used, it refers to the American style.
Standard reference, American style
According to the ‘American’ definition the prefix ‘s’ in sccm, slm or scfh refers to ‘standard’ conditions, 101.325 kPa absolute (14.6959 psia) and temperature of 0°C (32°F).
Please be aware of the reference conditions when ordering an instrument. ‘Normal’ and ‘Standard’ can be relative to each customer.
Why is this important? Because mixing up these reference conditions causes an offset in what the customer expects to see by greater than 7%!
Traditionally, and in most cases we see, dosing- or metering pumps are believed to be accurate because the theory is that a known pump head displacement will move a known volume over a known time giving a known delivered volume. In practice however it will never achieve a high level of accuracy with deviations of 10-15% being normal. Inaccuracies like this are caused by many changing process conditions, such as:
Wear of components
These factors can each be the cause of an inaccuracy in the expected volume of displacement from a pump head movement. If you then multiple each of those factors you can realise quite large measuring errors that create both inaccuracy and inconsistency.
Please refer to our earlier blog about ‘High Accuracy’.
What can be done to improve the accuracy?
Option 1) Add a flow meter between the pump and the process
By adding a flow meter between the pump and the process, you can take information from the flow meter to adjust the speed of the pump. Traditionally, this would be managed with an analogue output signal, 4….20 mA or similar, from the flow meter into a separate PID controller that compares the real flow signal to the desired flow. Subsequently, the electronic controller can then adjust the speed of the pump to achieve the desired dose or flow.
Using this solution will mitigate the issues in the original solution, however it introduces more:
Slow flow signal due to signal filtering in the PID controller
Slow pump response due to extra control relay
Increased complexity with extra components
Time to achieve stable flow can be long
Additional price of meter and PID controller
Option 2) Direct mass flow measurement with a flow meter with built in PID control
Now we need to discuss the next possible solution, using a direct Mass Flow measurement device with built in PID control
that can drive a pump to achieve the desired dose or flow.
With this solution you do not need to include the pump in the control system, just give a set point demand to the mass flow meter and it will drive the pump to achieve the desired dose or flow. This solution will give you several advantages, such as:
Direct mass flow control of the flow
Mass flow dosing is independent of temperature and pressure, in contrast to the volumetric dosing when only a pump is used;
Accurate delivery mitigating normal pump issues
Alarm functionality of low flow
Preventative maintenance based on pump performance over time
Consistent flow measurement based on actual not assumed numbers
Coriolis mass flow meter in modular dosing system
These advantages can be utilised in many different industries:
Anywhere that liquid is dispensed into a container that will require quality assurance, and commonly the quality control test is carried out on a small percentage of the vials to ensure general compliance. If you use a mass flow meter to control the dose you can achieve 100% QC checking of your product with reduced human input.
If you need to dose additives, performance chemicals or mix liquids then the ability to control the flow of the additive and know what that flow is can be a huge advantage to the outcome of the application.
Pump control can offer accurate dosing solutions for house hold chemicals like detergents and cleaning products.
In this blog, I would like to share an application of our flow instrumentation at one of our customers in which we needed to deal with high temperature and high pressure. This customer – an energy research organisation - investigates a catalysed chemical reaction of a mixture of hydrocarbon compounds.
Catalysts are being used to accelerate a chemical reaction without actually being consumed. So a small amount of catalyst is sufficient to obtain a large amount of reaction products.
Solid catalysts are often small, highly porous particles, with a large internal surface area in a small volume. This internal surface contains active sites on which the reaction takes place. Gaseous or liquid chemicals diffuse into the pores of these particles, and react at the catalytically active sites to reaction products that diffuse out of the particle. Often, these reactions occur at extreme process conditions.
What were the application requirements?
A simple and reliable solution had to be found to inject a liquid flow at a high pressure. This injection has to take place at 30 to 60 bars, and needs to result in a stable flow without pulsation. Furthermore, the liquid flow needs to be controlled accurately, and during the process it has to be known how much liquid actually has been injected.
Which solution did the customer choose?
The solution comprises a Coriolis mass flow meter that controls a HPLC piston pump at the inlet side of the reactor and an independently operating back pressure controller at the outlet side. The tested Coriolis mass flow meter (mini CORI-FLOW ML120) has proven to be a very stable and accurate mass flow meter. The WADose HPLC pump gives a very stable flow without pulsation. The combination of an HPLC pump and mass flow meter works as a mass flow controller. The control valve of the Coriolis mas flow meter is not necessary, as the pump is used as an actuator.
The pump can handle a liquid viscosity of max. 40 mPa.s at the upstream side. The maximum operating temperature is 70 °C. The temperature of the furnace that contains the reactor tube with small catalyst particles is much higher. The pressure at the reactor tube outlet has to remain at a high value. Beyond the outlet there is a cold trap for water or oil condensation, a back pressure controller with control valve that can handle pressure differences up to 400 bars and an exhaust to atmospheric pressure.
The pressure controller can handle gas and liquid in a very stable controlled flow. Especially at very small flow rates, this pressure controller has a much better control performance than a mechanical pressure reducer. The exhaust is used to remove gas that has been produced at the reaction.
The pump has three control modes: pressure, volumetric (only the speed of the piston is controlled) and mass flow. The latter is a special feature that can be offered, and is convenient from a chemist’s point of
view. As the flow can be controlled directly, the exact number of moles injected to the process is known.
Control and monitoring occurs via the digital interface. The mass flow measure and setpoint, density, temperature and counter value are visible via this single digital interface.
The success of this setup has been demonstrated by a recent order of three additional pumps.
For more details have a look at the application story 'Catalysis at high pressure'.
Within the medical arena there is increased pressure on budgets and financial accountability, with a significant trend for the sector to look again at how resources are used and where savings can be made.
One of the largest expenditures in most hospitals is the cost of purchasing or producing the various medical gases needed, such as Medical Air, Nitrogen, Oxygen and Nitrous Oxide. Often the usage and consumption of these gases is neither monitored nor measured or, whenever it is done, it is often a crude estimation, inaccurate and recorded only by pen and paper.
Most hospitals rely on the rate at which the cylinders (in which the gas is supplied) empty to determine the amount and rate of gas used. There are of course many issues associated with this method, such as:
The amount of gas in a particular sized cylinder can vary greatly, even when directly delivered by the gas supplier
Total gas consumption and peak times of consumption cannot be accurately determined
Leaks can go undetected
Specific point of use consumption is impossible to determine
This makes it very difficult to manage costs overall and to assign invoicing costs to individual departments and sections.
A company specialising in the design, installation and maintenance of gas systems was asked to install the medical gas network in a new hospital. An approach was made to Bronkhorst UK Ltd for the supply of gas meters which could then be communication-linked to the building maintenance system.
Thermal mass flow Instruments with integrated multi-functional displays were offered to fulfil both the accuracy and reliability requirements . As a result of their through-flow measurement (Constant Temperature Anemometry - CTA technology) the thermal mass flow instruments offered the additional benefits of no risk of clogging, no wear as there are no moving parts, minimal obstruction to the flow of the gas and hence ultra-low pressure drop, all based upon the fact that the instrument body is essentially a straight length of tube.
In addition to the local integrated displays both 4…20 mA and RS232 output signals were available ensuring integration with the Building Management System (BMS). This gave the end user real time continuous data logging and remote alarming should the gas supply enter low- or high-flow status for any given event. As a double failsafe the instrument offers both on-board flow totalization and further hi/lo alarms.
The installation of the mass flow instruments for this hospital application provided the following benefits to the client:
1. On primary networks:
Separated invoicing for hospital/clinic/laboratory departments sharing the same source of medical gas
Monitoring and acquisition of consumption data
Leak detection within gas line, safety vent and medical gas source
2. On secondary networks:
Independent gas consumption invoicing between the health institution departments
Monitoring and acquisition of consumption data
Leak detection within gas line
Subsequent installations across Europe have followed the trend of increased accountability by installing a Mass Flow Meter for the incoming bulk delivery, obtaining a totalized flow reading and cross matching this to the bulk invoice. This could be useful in the event of inadvertent errors or typos when a bulk delivery invoice is being raised.
Did you know that natural gas is odorless? I didn’t… I always find it having a penetrating sulfur scent. Well, it appears that this penetrating scent is added to the natural gas on purpose. Let’s see why this is.
As natural gas is combustible and odorless by nature, the government requires some safety measures here. Many countries have established safety regulations how to handle natural gas and which gas needs odorisation. This is mostly done by the Health and Safety department (HSE) of the local government.
What about natural gas odorisation?
Today’s question is about this subject. Why does gas smell when it is odorless by nature? This is the point where gas odorisation comes in.
Odorisation of natural gas is done to act as a ‘warning agent’ in case of leakage. The idea is that people can smell the gas prematurely if it is present. Because, if there is too much gas present it can be explosive.
As shown in the picture, the LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) are crucial here. If the concentration of the combustible substance present in the air is too low (< LEL), than no combustion will occur. It the mixture is too rich (> UEL), there is a huge amount of gas in the air and only partial combustion will occur. Gases become dangerous in between the LEL and UEL. Therefore, it is most important for people in the surroundings to smell the gas in time, before the concentration is too high and it exceeds the LEL.
As a result, it is stated in the safety regulations that natural gas has to be detectable at a concentration level of 20% of the LEL and this is done by odorisation. Needless to say that the odor used in the gas is not dangerous to people’s health.
When is an odor added to gas?
This depends on the type of gas line. We know ‘distribution lines’ and ‘transmission lines’.
Distribution lines are local natural gas utility systems that include gas mains and service lines, such as the commercial gas used at domestic environments. All these distribution lines need to be odorised. For the transmission lines it is stated in the regulations when to odorise it.
Picture LEL and UEL
For the odorisation there are many different odorants available, such as Tetrahydrothiophene (THT) and Mercaptan. Selecting the odorant depends on the properties of the gas to be odorised, pipeline layout, ambient conditions etc.
Tetrahydrothiophene or THT is a well-known odor. THT is under ambient conditions a colourless volatile liquid with an unpleasant smell.
Controlled supply of THT using mass flow controllers
Bronkhorst had the pleasure of developing a solution for a Dutch customer to add THT to their biogas. Biogas was generated from anaerobic decomposition of organic matter and upgraded to natural gas quality to inject into the Dutch natural gas main. As commercial natural gas in the Netherlands has to contain at least 18mg of THT per cubic meter gas, the process of adding this to the commercial gas had to be done really accurately.
ATEX Zone 1 Coriolis Mass Flow Meter
The traditional approach to add THT is using a pump with a fixed stroke volume. However, low gas flow rates using a pump for batch-wise injection may lead to liquid THT remaining in the gas lines. THT may not be mixed well with the gas and might have the wrong concentration. A homogeneous injection of THT is therefore much better. Besides this, THT is a relatively expensive odor which also makes an accurate injection very much desired.
A better solution here would be using a combination of a pump with a Coriolis mass flow controller, in our case the mini CORI-FLOW™ series mass flow controllers. The Coriolis instruments make it possible to dose both continuously as well as accurately.
Something to be taken into account is the classification of the area. As gases in principle are explosive, it is very common for the environment around gases to be classified as a hazardous area. Most common classifications (in Europe) are marked as ATEX zone 1 or zone 2. Just make sure to select the right material to use.
For solutions such as THT odorisation processes, Bronkhorst can offer both ATEX/IECEx zone 1 and zone 2 solutions. Our mini CORI-FLOW Exd mass flow meter, for zone 1 applications, is a collaboration with one of world’s leading manufacturers in explosion protection, Electromach member of the R.STAHL Technology Group.
Thermal mass flow instruments that make use of a bypass (capillary bypass or bypass sensor) are what most people have in mind when they think of thermal mass flow instruments. What are the differences?
In instruments based on the thermal principle, power is applied to heat the sensor tube. Accordingly the temperature of the tube is measured at two points. With no flow measured, the temperature differential between the two points will be zero.
When the flow increases, the temperature at the first measuring point will decrease, as fluid carries away the heat. At the same time the temperature at the second measuring point will increase as the fluid carries heat to it. More flow will result in a greater temperature differential and this temperature differential is proportional to the mass flow.
Another technology used to measure mass flow is CTA (Constant Temperature Anemometry).
In a CTA (through flow, straight tube) instrument there are two measurement “probes” inserted into a straight tube flow path. The first “probe” both heats and measures the temperature of the fluid, as the second “probe” measures the temperature of the fluid.
Again, as the gas flow increases the gas will carry heat from the first measuring point to the second one. In a CTA, however, the power is varied to keep the temperature between the two measuring points constant, and it is this power level that is proportional to the mass flow.
Each technology has its advantages and disadvantages which generally are application specific.
A clean, dry gas application where higher accuracy is as important as repeatability, may be a better application for a bypass instrument like the Bronkhorst EL-FLOW series.
An application with a dirty or slightly moist gas, or where lower accuracy but high repeatability and robustness is required, may be a better application for a CTA instrument like the Bronkhorst MASS-STREAM™ series.