Thursday, February 28, 2019

Problems with Measuring Level of High Density Pulp Stock

Level Measurement in Pulp Stock
Level Measurement in Pulp Stock has posed problems for a wide range of level measurement technologies. In most pulp mills this is the most challenging application on site.

The environment in the high-density pulp tank is very corrosive to most common metals and the clouds of dense steam vapors that rise from the stock and foaming surface of the level are a constant source of signal loss for many non-contact technologies.

Due to the size of the vessel and density of the stock, mechanical devices are typically short  lived.Stock coatings that form on all interior surfaces add additional mechanical stress and the pulp coating deposits can adversely affect accurate level measurement performance.

One of the challenges of a level measurement device is to control the level in the stock tank, thereby helping to control the average pulp density by controlling the speed of the pump.The response time of the level measurement system is critical in controlling the pump speed and reducing pump oscillations and surges that will eventually reduce the life of the pump.Too slow of a response from the level instrument will allow the level to rise above it ’s optimum density level and cause excessive loading and wear on the pump.

Piping Specialties, Inc.
PSI Controls

Saturday, February 23, 2019

Types of Pneumatic Valve Actuators

Scotch-yoke actuators
Scotch-yoke actuators (Morin)
Pneumatic valve actuators all provide the same function:  They convert air pressure to rotational

movement and are designed to open, close, or position a quarter-turn valve.  These include ball valves, plug valves, butterfly valves, or other types of 90 degree rotational valves.

The basic design variations of pneumatic valve actuators are as follows:

  • Rack and pinion
  • Scotch-yoke
  • Rotary vane

Let's review each of these in detail:

Rack and Pinion Actuators

Rack and pinion actuator
Rack and pinion actuator (Unitorq)
These actuators are sometimes referred to as, “lunch box,” because they, well, look like a lunch box. This actuator uses opposing pistons with integral gears to engage a pinion gear shaft to produce rotation. They are usually more compressed than a scotch yoke, have standardized mounting patterns, and produce output torques suitable for small-to-medium sized valves.  Rack and pinion nearly always include standard bolting and coupling patterns to directly attach a valve, solenoid, limit switch or positioner.  One of their features include several smaller coil springs mounted internally, which provide the torque to return the valve to its starting position.

Scotch-yoke Actuators 

Scotch-yoke actuators
Scotch-yoke actuators internal view.
These actuators come in a multitude of sizes, but are usually used on larger valves because they can produce a very high torque output.  They employ a pneumatic piston mechanism to transfer movement to a linear push rod.  That rod, in turn, engages a pivoting lever arm to provide rotation. Spring return units have a large return spring module mounted on the opposite end of the piston mechanism working directly against the pressurized cylinder.

Rotary Vane Actuators 

Rotary vane actuators
Rotary vane actuator animation.
These actuators are usually used when the application requires a significant space savings.  They take up less space when comparing size-to-torque with rack and pinion and scotch yoke. Rotary van actuators also benefit from a reputation of longevity.  They contain fewer moving parts than other types of pneumatic valve actuators.  Rotary vane actuators use externally mounted, helically wound "clock springs" for their spring return mechanism.

These style of valve actuators can all be secured with direct acting or spring return versions. Direct acting actuators use the air supply to move the actuator in both directs (open and close). Spring return actuators, as the name describes, uses springs to move the actuator back to its "resting" state. Converting a version from direct acting to spring return is done through simple modifications, typically just adding an external spring module, or removing the end caps from rack and pinion actuators and installing several coil springs.

When considering the choice of pneumatic valve actuators, your decision comes down to size, power, torque curve and the ease of adding peripherals. To ensure that your valve actuation package will be optimized for safety, longevity, and performance, the advice of a qualified valve automation expert should be sought out. That expert will be able to help you with the best selection of the appropriate valve actuator for any quarter turn valve application.

For more information on valve actuation, contact Piping Specialties, Inc.

Tuesday, February 12, 2019

Understanding Differential Flow Measurement

The differential flow meter is the most common device for measuring fluid flow through pipes. Flow rates and pressure differential of fluids, such as gases vapors and liquids, are explored using the orifice plate flow meter in the video below.

The differential flow meter, whether Venturi tube, flow nozzle, or orifice plate style, is an in line instrument that is installed between two pipe flanges.

The orifice plate flow meter is comprised the circular metal disc with a specific hole diameter that reduces the fluid flow in the pipe. Pressure taps are added on each side at the orifice plate to measure the pressure differential.

According to the Laws of Conservation of Energy, the fluid entering the pipe must equal the mass leaving the pipe during the same period of time. The velocity of the fluid leaving the orifice is greater than the velocity of the fluid entering the orifice. Applying Bernoulli's Principle, the increased fluid velocity results in a decrease in pressure.

As the fluid flow rate increases through the pipe, back pressure on the incoming side increases due to the restriction of flow created by the orifice plate.

The pressure of the fluid at the downstream side at the orifice plate is less than the incoming side due to the accelerated flow.

With a known differential pressure and velocity of the fluid, the volume metric flow rate can be determined. The flow rate “Q”, of a fluid through an orifice plate increases in proportion to the square root the pressure difference on each side multiplied by the K factor. For example if the differential pressure increases by 14 PSI with the K factor of one, the flow rate is increased by 3.74.

Piping Specialties, Inc. / PSI Controls