Pressure transmitter is crucial to any wastewater treatment process.
Environment protection authorities have a very tight tolerance for contaminants present in the water we discharge into natural water bodies. Therefore, wastewater treatment plants worldwide invest significant efforts and resources in ensuring the efficiency of all their equipment, including pressure transmitters.
However, a wide range of options is available in pressure transmitters for sewage treatment.
Therefore, choosing an exemplary pressure transmitter for a specific application is critical to ensuring that the device operates accurately.
This article will discuss some tips to streamline pressure transmitter selection for sewage treatment. But before that, it is essential to understand the working of a pressure transmitter.
What is a Pressure Transmitter?
A pressure transmitter is a sensor that senses the system’s pressure and converts it into electrical signals. These devices have a diaphragm that is in contact with the system pressure. The system pressure exerts mechanical strain on the diaphragm.
The sensor converts this mechanical strain into electrical signals, which the transmitters send to the controller, taking action when needed.
Now, saying a pressure transmitter is a pressure sensor redundant. However, a pressure sensor is a generic term that people use for all the devices that convert pressure variations into electrical signals. In contrast, a pressure transmitter amplifies these signals and transmits them to an often-far-off receiver.
Because it communicates with far-off controllers, transmitters usually output electrical signals in 4-20 mA. That is because current signals offer good immunity against electrical noise.
What to Look For When Choosing a Sewage Treatment Process Pressure Transmitter?
Sewage treatment application is one of those industries that put the process equipment through the harshest of the environment and the most challenging operating conditions.
The challenges of sewage treatment are compounded by the fact that they rely on various methods to meet their effluent specifications. These include biological treatment, thermal treatment, ultrafiltration, and reverse osmosis.
The industry also relies on an extensive network of pipes to transport the water from where it comes into contact with sewage to where it needs to be treated before being released back into waterways or reused as drinking water.
A sewage treatment facility must withstand constant exposure to toxic water, possibly from the ground or other sources. It must operate reliably and accurately, even when exposed to vibrations and shocks that would otherwise cause damage.
In addition, these facilities must monitor their effluent quality data and maintain records of all maintenance activities performed on their systems to ensure compliance with national standards set by regulatory bodies.
1. The Pressure Range
Different pressure transmitters work with other operating pressure ranges. Therefore, using a pressure transmitter in an application where the operating pressure is beyond or below the device’s measurement range can lead to inaccuracies.
So, before you pick a pressure transmitter for your sewage treatment process, consider the operating pressure range. That means you must estimate the minimum and maximum pressure values the system is likely to reach and then get a pressure transmitter with an operating range wide enough to accommodate the variations in your system pressure and keep working accurately.
2. The Type of Pressure Sensor
There are three primary types of pressure transmitters. These are Absolute, Gauge, and Differential pressure transmitters.
Absolute pressure transmitters take the change in atmospheric pressure and measure it against a perfect vacuum.
On the other hand, pressure gauges measure pressure relative to atmospheric pressure.
Differential pressure sensors sense the difference in pressure across two points and relay their difference.
Each of these types of pressure sensors is suitable for various applications. However, the most common type of pressure transmitter for sewage treatment facilities is likely to be a differential pressure transmitter. Differential pressure transmitters use a piezoelectric crystal to detect pressure differences between two system parts. The crystal’s orientation determines whether it senses positive or negative pressure. This transmitter can accurately measure the pressure across any number of pipes, lines, or other components within a system.
3. The Accuracy of the Pressure Transmitter
A pressure transmitter’s accuracy depends on three factors: Repeatability, Linearity, and Hysteresis.
The repeatability of a pressure transmitter refers to how well its output is repeatable when measured over time or under varying conditions.
A pressure transmitter’s linearity refers to how closely its output matches the actual pressure that it measures.
The hysteresis of a pressure transmitter refers to how much the output varies as a function of changes in the input.
Most pressure transmitters exhibit an accuracy of 0.5% to 0.05% of the full operating range. Therefore, you must determine how tightly you need to control the system’s pressure, flow, or level variables and choose a pressure transmitter accordingly.
4. The Operating Temperature Range
Knowing how a pressure transmitter works can be tricky, especially if you don’t know much about them.
A pressure transmitter is often sensitive to extreme temperatures. For example, if you’re using a pressure transmitter in a hot environment and operating at room temperature, its temperature may be too high. The result: inaccurate readings. If you’re using a pressure transmitter in a cold environment and operating at room temperature, its temperature may be too low. Hence, a too-high or too-low operating temperature value can compromise the pressure transmitter’s accuracy.
It is implausible that a wastewater treatment pressure transmitter will operate at extremely high temperatures. However, significant variations in the operating temperatures of a treatment plant can be expected.
It is therefore conceivable to invest in a pressure transmitter that can withstand fluctuations in system pressure without resulting in a decrease in accuracy.
5. Restrictor Pin
A restrictor pin in a pressure transmitter is a small arrangement inside the device that dampens pressure spikes in the water. It does this by reducing the internal diameter of the line where the sensor is located.
A pressure transmitter for wastewater treatment is a device that is almost always exposed to shocks and vibrations due to variations in the flow of incoming water and the presence of large solid bodies. And frequent high-pressure spikes can defer the device’s accuracy or contribute to its premature failure.
Consequently, it is advisable to opt for a pressure transmitter with a restrictor plug when considering sewage treatment. The system sends out shocks and vibrations, so the components must be resilient.
6. Adaptability to pH Levels
Various impurities affect the pH of waste water entering a treatment plant.
If the pressure transmitter is not built to sustain an acidic or alkaline influent’s corrosive nature, it will likely erode internally and externally. Therefore, when purchasing a pressure transmitter for wastewater treatment, ensure that it is protected from erosion and wear by system fluids. Furthermore, ensure the transmitter’s sensing element is made of 316L Stainless Steel, as it offers good resistance against corrosive liquids.
7. Compatibility with Process Fluids
Different pressure transmitters are manufactured to measure the pressure of various fluids. For example, some transmitters are used to measure air pressure, while others are used to sense oil pressure. In comparison, some pressure transmitters measure water pressure.
For this reason, make sure your pressure transmitter is compatible with the wastewater it will treat.
8. Protection Against Moisture Intrusion
You want to protect a pressure transmitter that you’ll use in wet environments, like a wastewater treatment plant. That way, moisture won’t get into the sensor and cause damage. When you purchase one, make sure it has an IP rating and NEMA rating that is adequate for your needs.
Submersible pressure transmitters can be used in sewage treatment facilities to measure levels. When using a transmitter in these situations, it is recommended that it be rated IP68 or NEMA 6P.
9. Sensing Element
A pressure transmitter uses three types of sensing elements. These are resistive, capacitive, and piezoelectric sensing elements.
The resistive sensor works on the piezoresistive principle, producing a change in resistance when there is a change in the pressure applied. It can measure very small changes and has excellent long-term stability but are not resistant to shocks or vibrations.
Capacitive sensors work by measuring electrical signals when an object is placed onto a capacitor. These sensors are not susceptible to dust, liquids, and humidity in the same way that optical or mechanical sensors are. Capacitive sensors are also robust, operate well over a wide range of temperatures, and are resistant to short-term overpressure.
Piezoelectric sensors generate an electrical charge in response to mechanical stress, or changes in pressure. These highly robust sensors can withstand the harshest temperatures while featuring a wide range of operating temperatures.
After understanding the different features of pressure sensors, a piezoelectric or capacitive sensor is best suited to sewage treatment.
Conclusion
A pressure transmitter for wastewater can be exposed to harsh conditions when it is used in a sewage treatment plant. This includes corrosive wastewater and less-than-ideal operating environments. To make sure you get the most reliability out of your transmitter–even in the most extreme working conditions–choose one that has been certified for such applications.
If you want to know which pressure transmitter is right for your system, consider what it will be used for. And make sure the transmitter can handle any corrosiveness, moisture ingress, shocks, or vibrations.
