May 19, 2015
Many people seem to think that all plunger sensors are the same. But in reality, that statement couldn’t be more wrong. There are substantial differences in all of the sensors available today. Understanding these differences and making an informed decision could end up saving you thousands of dollars per well.
The plunger arrival sensor is often the least expensive piece of the plunger lift system, but it has the ability to cost you the most. Sensors have been notoriously unreliable in the past. Not only can a bad sensor lead to missed arrivals or premature shut-ins, but your controller could end up making the wrong decisions when it comes to optimizing the well if the sensor’s data is incorrect. Knowing exactly when the plunger arrives and never false detecting is key to a reliable and consistent plunger lift well.
Here are some things that you need to consider when evaluating plunger arrival sensors:
Many sensors are built for a single voltage (typically 6V, 12V or 24V). This means that you need to use a different sensor for your stand-alone controllers and remote terminal units (RTUs) because they typically provide different voltages. Try to select a sensor that supports any voltage from 5V – 24V and you should be covered. This means that if you are keeping some spare sensors on hand, you don’t have to stock as many because you can use them in any system.
There are so many different types of wells, lubricators, and plungers that it is impossible to have a solution that works for every scenario. Having the ability to adjust the arrival sensor’s sensitivity can help make plunger detection much more reliable. If you are having issues with a sensor picking up noise or vibrations and you are using a ferrous plunger, simply reduce the sensitivity to filter out the noise. When using a non-ferrous plunger, you can pick up the movements of the anvil or spring by increasing the sensitivity.
Some plunger sensor manufacturers encourage you to install their sensor at the bottom of the lubricator. This is because their sensor needs the plunger to pass by the sensor completely to detect. If you use a sensor that measures the magnetic field and is sensitive enough, this issue goes away. You can then install the sensor higher on the lubricator, on the master valve, or even on the casing. These sensors can often be configured such that the plunger can be detected several feet away from the sensor, compensating for wells where the plunger stalls before fully entering the lubricator.
Ensuring that you select a sensor that is appropriately certified is essential. If a well head can have gas present under normal operating conditions (i.e. surface casing vent), then it should be considered Class I, Zone 1 (Division 1). This has often been overlooked. Choose a sensor that can be installed in any area. If the sensor is rated Class I, Zone 0 (Division 1), it is rated for the most hazardous of locations.
One thing to keep in mind is whether the controller is rated for a less hazardous area. If that is the case, you may need to install a barrier. Another option is choosing a plunger arrival sensor that is explosion proof, which eliminates the need for a barrier.
What do you do when your plunger sensor doesn’t detect when it should? Maybe you try a different position or adjust the sensitivity. If it still doesn’t work, then you may just have to replace it.
If you select a sensor that has the ability to show you what it sees in real time, you can more quickly identify the issue and likely correct it. Furthermore, if it has a built in self-test, you can tell if the sensor is working properly so you don’t end up discarding a sensor that is still good.
The speed of the plunger is something that has historically caused endless problems. If the sensor uses an inductive coil, the plunger has to travel at a fast enough speed to be picked up. More current sensors look at the magnetic field in all directions around the sensor. These sensors just look for a change in the magnetic field, no matter how fast the change occurs.
Lightning can cause hundreds of wells to go down in a single night. The interference generated by nearby electrical storms is picked up by coil based sensors and passed on as hundreds or thousands of arrivals, crashing automation systems. Using a digital sensor with integrated filtering will ignore the interference from lightning and prevent mass outages saving operator time and lost production.
There is a common misconception that two wire sensors can simplify the installation process. The problem with these sensors is that they have an internal battery that is constantly being drained. Whether these are sitting somewhere in the supply chain, at your shop, or on a well, the life is draining out of the product. What’s even worse is that these batteries are not replaceable. Once it dies, you are throwing away hundreds of dollars. This is why it’s important to have a sensor that has a third wire to deliver power from your controller. This will ensure that the sensor never runs out of power and isn’t a disposable component.
Yes, sensors have software in them too. Often the embedded software is programmed into the sensor in the factory and cannot be changed. Only a couple of sensors allow you to communicate with them and upgrade the software. If improvements are made to the software and you can’t upgrade previously installed sensors, your only recourse will be to replace them.
When using titanium, stainless steel, or aluminum plungers your options are rather limited. These non-ferrous plungers are invisible when it comes to magnetics so often they can pass by without being detected. You could use a sensor that picks up vibration, but these can often trip when you don’t want them to. Instead, look for a plunger arrival sensor that is sensitive enough to pick up the movements of the anvil when the plunger reaches the surface.
Plunger sensors that are sealed in plastic cases, attach to the well head with cable ties or rubbed O rings, or that don’t have a strain relief for the cable are bound to fail. Why not invest in something that will last a lot longer? Pick a sensor with aluminum housing that has the ability to be used with standard hose clamp. It won’t be crushed if you tighten things too much and will likely come with a standard size port that allows you to install a cable gland or armored cable connector.
Price is always a factor, but should be of the least concern in this case. At most, sensors will vary a couple hundred dollars in price. A good sensor that detects every arrival and never needs to be replaced will easily pay for itself in short order. Remember, your time to troubleshoot sensor issues and to replace them time and again is worth something.
