Hydrostatic Pressure Measurement for Determining Liquid Level

Pressure transmitters can be used to provide liquid level in
pressurized or open vessels.
Photo courtesy Azbil North America

Pressure measurement is an inferential way to determine the height of a column of liquid in a vessel in process control. The vertical height of the fluid is directly proportional to the pressure at the bottom of the column, meaning the amount of pressure at the bottom of the column, due to gravity, relies on a constant to indicate a measurement. Regardless of whether the vessel is shaped like a funnel, a tube, a rectangle, or a concave polygon, the relationship between the height of the column and the accumulated fluid pressure is constant. Weight density depends on the liquid being measured, but the same method is used to determine the pressure. 

A common method for measuring hydrostatic pressure is a simple gauge. The gauge is installed at the bottom of a vessel containing a column of liquid and returns a measurement in force per unit area units, such as PSI. Gauges can also be calibrated to return measurement in units representing the height of liquid since the linear relationship between the liquid height and the pressure. The particular density of a liquid allows for a calculation of specific gravity, which expresses how dense the liquid is when compared to water. Calculating the level or depth of a column of milk in a food and beverage industry storage vessel requires the hydrostatic pressure and the density of the milk. With these values, along with some constants, the depth of the liquid can be calculated.

The liquid depth measurement can be combined with known dimensions of the holding vessel to calculate the volume of liquid in the container. One measurement is made and combined with a host of constants to determine liquid volume. The density of the liquid must be constant in order for this method to be effective. Density variation would render the hydrostatic pressure measurement unreliable, so the method is best applied to operations where the liquid density is known and constant.

Interestingly, changes in liquid density will have no effect on measurement of liquid mass as opposed to volume as long as the area of the vessel being used to store the liquid remains constant. If a liquid inside a vessel that’s partially full were to experience a temperature increase, resulting in an expansion of volume with correspondingly lower density, the transmitter will be able to still calculate the exact mass of the liquid since the increase in the physical amount of liquid is proportional to a decrease in the liquid’s density. The intersecting relationships between the process variables in hydrostatic pressure measurement demonstrate both the flexibility of process instrumentation and how consistently reliable measurements depend on a number of process related factors. 

Share your process measurement and instrumentation requirements and challenges with professionals that specialize in their proper selection and application. Combining your own process knowledge and experience with their product application expertise will help to develop effective solutions.

Commercial and Industrial Process Heating Methods

Electric heating element of special design

Many industrial processes involve the use of heat as a means of increasing the energy content of a process or material. The means used for producing and delivering process heat can be grouped into four general categories.

• Steam
• Fuel
• Electric
• Hybrid

The technologies rely upon conduction, convection, or radiative heat transfer mechanisms, solely or in combination, to deliver heat to a substance. In practice, lower temperature processes tend to use conduction or convection. Operations employing very high temperature rely primarily on radiative heat transfer. Let's look at each of the four heating methods.

STEAM

Steam based heating systems introduce steam to the process either directly by injection, or indirectly through a heat transfer device. Large quantities of latent heat from steam can be transferred efficiently at a constant temperature, useful for many process heating applications. Steam based systems are predominantly for applications requiring a heat source at or below about 400°F and when low-cost fuel or byproducts for use in generating the steam are accessible. Cogeneration systems (the generation of electric power and useful waste heat in a single process) often use steam as the means to produce electric power and provide heat for additional uses. While steam serves as the medium by which heat energy is moved and delivered to a process or other usage, the actual energy source for the boiler that produces the steam can be one of several fuels, or even electricity.

FUEL

Fuel based process heating systems, through combustion of solid, liquid, or gaseous fuels, produce heat that can be transferred directly or indirectly to a process. Hot combustion gases are either placed in direct contact with the material (direct heating via convection) or routed through tubes or panels that deliver radiant heat and keep combustion gases separate from the material (indirect heating). Examples of fuel-based process heating equipment include furnaces, ovens, red heaters, kilns, melters, and high-temperature generators. The boilers producing steam that was described in the previous section are also an example of a fuel based process heating application.

ELECTRIC

Electric process heating systems also transform materials through direct and indirect means. Electric current can be applied directly to suitable materials, with the electrical resistance of the target material causing it to heat as current flows. Alternatively, high-frequency energy can be inductively coupled to some materials, resulting in indirect heating. Electric based process heating systems are used for heating, drying, curing, melting, and forming. Examples of electrically based process heating technologies include electric arc furnace technology, infrared radiation, induction heating, radio frequency drying, laser heating, and microwave processing.

HYBRID

Hybrid process heating systems utilize a combination of process heating technologies based on different energy sources or heating principles, with a design goal of optimizing energy performance and overall thermal efficiency. For example, a hybrid steam boiler may combine a fuel based boiler with an electric boiler to take advantage of access to low off-peak electricity cost. In an example of a hybrid drying system, electromagnetic energy (e.g., microwave or radio frequency) may be combined with convective hot air to accelerate drying processes; selectively targeting moisture with the penetrating electromagnetic energy can improve the speed, efficiency, and product quality as compared to a drying process based solely on convection, which can be rate limited by the thermal conductivity of the material. Optimizing the heat transfer mechanisms in hybrid systems offers a significant opportunity to reduce energy consumption, increase speed and throughput, and improve product quality.

Many heating applications, depending on scale, available energy source, and other factors may be served using one or more of the means described here. Determining the best heating method and implementation is a key element to a successful project. M.S. Jacobs and Associates specialize in electric heating applications and facets of the industrial production of steam. Share your process and project challenges with them and combine your facilities and process knowledge and experience with their product application expertise to develop effective solutions.

Wireless Communications Deliver Real Time Process Data From Remote Operating Sites

Oil is where you find it, with many prospecting and production sites located where the communication options taken for granted in developed areas do not exist. Oil is big and serious business, with tremendous sums of money at risk on the prospect of reaping even greater returns. Every business operation, though, is of great importance to the stakeholders. Countless operations in little known industries and endeavors are located beyond the boundaries of modern communications infrastructure.

If you want a data connection, bring your own.

Remote operating sites, whether for oil extraction or other purposes, will often be automated. Some decision making system or individual is responsible for the safe and effective operation of the remote site, or has a use or need for real time data being gathered at the remote site. Radio transmission is a viable, maybe the best, option for delivering real time data from a remote site to a central office.

• Transmission options for 900 MHz, 2.4 GHz, cellular, and satellite systems are readily available.
• Equipment operates on low voltage, low power. Suitable for solar or other remote site power source.
• No special instrumentation needed. Radio transmitting and receiving equipment interfaces directly with analog signals from common industrial process transmitters.
• No "across the land" cabling needed.
• Equipment can be configured to resist extreme environmental conditions.

Analynk manufactures transmission and receiving equipment that builds the bridge between remote sites and the home office. From elemental componentry to integrated, ready to run systems, Analynk specializes in wireless communications for industrial process control. Share your wireless process data connection challenges with process measurement and control specialists. Whether an expansive multipoint, or a single point application, application specialists can combine standard or customized products into a practical solution for every application.

Zero Bleed Pneumatic Controller for Valve Actuators and Other Process Control Apps

This video shows the components of the BiFold Zero Bleed Pneumatic controller. Called "PICO", this unit was first described in a previous article. Check out the video, as it nicely lays out the various operating components of a complete system. More information is available from product application specialists, with whom you should share your valve control and actuation challenges to get positive and effective solutions.

Zero Bleed Pneumatic Controller

The PICO consists of a single logic control head
and a digital filter booster
Courtesy Bifold - Rotork

The Bifold brand, under the Rotork corporate umbrella, developed a true zero bleed pneumatic position controller for valve actuators. The product, called PICO, consists of a single logic control head and a digital filter booster. The control head unit provides bluetooth communications, ESD monitoring and control, graphic display, integral valve feedback measurement, low power modes, a partial stroke test feature and local control setting switch. The properly installed assembly is suitable for use in hazardous locations.

The new control unit is capable of fulfilling applications employing positional control, on/off and ESD (emergency shutdown) valves. The filter booster allows the small size of the PICO to deliver the flow rate of a substantially larger system of conventional design.

The PICO, with control head and filter booster
shown installed on pneumatically actuated valve
Courtesy Rotork - Bifold

The PICO provides a number of operational benefits to pneumatic actuated valve applications. More information is available from product application specialists, with whom you should share your valve control and actuation challenges to get positive and effective solutions.

Two New Products From SVF Flow Controls

New products from SVF Flow Controls

SVF Flow Controls, manufacturer of valves and actuators for industrial process control (learn more here), recently released their new rack and pinion actuator, the EZ-Tork™. The line of pneumatic valve actuators features bi-directional stroke adjustment, a continuous position indicator, hard anodized aluminum housing, and universal mounting. The line offers 33 models spanning a broad range of operating torque. Units are available in double acting or spring return variants.

The SL Series of butterfly valves feature direct mounting for electric or pneumatic automation. Manually operated units have a ten position locking handle or gear operator. Epoxy coated ductile iron body, 316 stainless steel disc, and EPDM or BUNA seats enable the application of this valve throughout many industrial settings. Sizes range from 2" to 12".

More detail is available from valve and fluid control specialists. Share your application challenges and combine your own process knowledge with their product application expertise to develop effective solutions.

Prevent Condensation in Your Facility

Condensation can have a negative impact in
plants, buildings and other and facilities

Condensation, the accumulation of liquid water on a surface through contact with humid air, can be harmless in some settings, an undesirable or even damaging occurrence in others. In situations where condensation is undesirable, taking steps to prevent the conditions that preclude its formation are relatively simple and deliver a good payback.

What is condensation? In general usage, the term refers to the formation of liquid water droplets that occurs when humid air contacts a cooler surface. It is the liquid moisture that accumulates on the exterior of a glass containing a cold drink. Properly, the term condensation names the process of a vapor changing to a liquid. It is the opposite of evaporation. Condensate (note the different word form) is the liquid accumulated through the condensation process. This article is limited to condensate that forms when atmospheric air contacts a cold surface, so the general usage term condensation will be used.

Where can it happen? Water vapor is contained in air when it has sufficient energy to remain in the vaporous state. Remove some of that heat energy and a calculable quantity of the water vapor will no longer be supported, condensing into liquid water. The temperature at which any given quantity of air will start to shed some of its water vapor content is primarily determined by the concentration of water vapor in the air. A higher water vapor content will result in a higher temperature at which the water vapor will begin to condense. In everyday terms, higher relative humidity leads to a higher temperature at which condensation takes place.

What is the range of impact? Condensation appears to us as water that almost magically manifests on a surface. It seems to come right out of thin air.....because that is where it came from. It can form locally or broadly throughout an area. The potential impact of condensation arises from the fact that it is liquid water. Anything that will be damaged by water will be adversely impacted by condensate formation on its surface. This includes rust and corrosion of metals, spotting on material or  object surfaces, the promotion of mold and mildew, and a wide range of other undesirable effects. Accumulated condensate on overhead objects or surfaces can eventually drip onto equipment, materials, and work areas situated below. Puddles of water on a floor can also create a hazard.

Prevention is the best, maybe the only cure.

How to prevent condensate formation?

• Ventilation - If there is a source of moisture in a space that is elevating the humidity, continually diluting the space moisture content by introducing fresh air with a lower moisture content may be an effective prevention method. Ventilation relies on the fresh air conditions always being sufficient for moisture reduction without creating some other adverse impact on the space. For example, ventilating with outdoor air may be effective throughout only part of the year. Without a reliable source of ventilation air with known conditions, this method may not always deliver the desired results. Ventilation is an active method that requires energy to move the ventilation air. Additional energy may be required to adjust the temperature or moisture conditions of the ventilation air, as well.
• Insulation - The surfaces where condensation occurs can be isolated from the moist air by insulating materials. This is common with HVAC ductwork and process piping. If done properly, this method is effective. The goal is to create a new surface that does not exhibit the cooler temperatures of the isolated surface. The thickness and reduced thermal conductivity of the insulation material will achieve this. There is also a vapor barrier on the exterior of the insulation that prevents entry of moisture laden air into the insulation material. It is important the the vapor barrier installed as part of the insulating process remain intact and undamaged. Otherwise, water vapor will enter the insulating material and condense, with the potential for a localized failure of the insulating scheme. Insulation is a passive measure that requires no added energy to remain effective.
• Dehumidification - Outright reduction of moisture contained in the air of an enclosed space will reduce the temperature at which water vapor condenses. Dehumidification machinery is available in a wide range of sizes and performance levels to suit almost any scenario. Though it requires energy to operate, the machinery is generally simple and operates automatically to maintain a space condition that will not support condensation.
• Heating - Some cases can be most effectively treated using the application of a small amount of heat to the surface where condensation forms. This active method can be very effective when the need is localized. Also, surface heaters can be fabricated that will fit where insulation will not, and the heating assemblies may be more resistant to impact and damage than insulating materials. Proper control of heating equipment will minimize energy consumption.

Implementing an effective plan to combat condensation involves the identification of the conditions that promote its formation in your own facility. Selecting the best prevention plan calls for consideration of costs and reliability of various schemes. Active methods, such as heating or dehumidification, have some capacity for adjustment if conditions change over time. Insulation plans should have sufficient headroom or safety factor in their design to accommodate unforeseen conditions.

Reach out to product application specialists and share your challenges and concerns. Combining your own facilities and process knowledge with their product application expertise will result in effective solutions.