Gas Metering with Honeywell Transmitter

Gas metering is a big part of what we offer customers. One product that has proven to be very successful is the Honeywell Smart Multivariable (SMV) Transmitter.  It is an industry leader – there are no others on the market that work as well.  It measures the gas pressure & temperature and measures a differential pressure across a primary element.  The primary element can be an orifice plate or a Preso Ellipse pitot tube.  It creates a differential pressure that we measure and relate to flow (the square root of the pressure drop is proportional to the flow rate).  The SMV calculates a corrected flow by accounting for the compressibility of the gas along with the pressure and temperature that exist in the pipe.  It is quite an involved calculation that is pre-loaded into the transmitter and set up using a software wizard.  Please keep it in mind for gas metering applications – it is an affordable alternative.  For under $4000 you can measure big flows accurately with easy installation. [Read more...]

Not Just the Hard to Find Parts

Many people think of Stromquist when it comes to buying those “hard to find” parts — the old or obsolete parts or those that just aren’t very common. We’re well known for our great counter and inside sales staff who find matches and replacements for such items. While we do appreciate that you think of us for these, we want you to also think of us for those everyday, run of the mill parts like thermostats, valves, gauges and sensors. Some people automatically go to the bigger supply house chains for these types of parts and only come to Stromquist for the more elusive items. So we would like to ask that you give us a try on your “regular” parts too. We stock a wide range of Honeywell, Johnson Controls, Schneider Electric (formerly TAC/Invensys), Belimo, White Rodgers and many more. And we’re competitive on our pricing. The next time you need that simple thermostat, a relay or pneumatic fittings, give us a call. We’d be happy to pull one off the shelf for you in addition to finding the replacement for that rusty part that’s 25 years old.

Understanding: On/Off, Floating, Modulating/Proportional Control

First, to understand these types of control you must have the elements of control. The elements of control are the sensor (senses the medium being controlled), the controller (device either preset or programmed to react to the sensor), and the final controlled device such as a damper or a control valve (receives input signals from controller to affect change in controlled medium). These elements are considered the control loop.

On/Off control is the basic type of control in a control loop.  With On/Off control, the sensor senses the controlled medium and sends a signal back to the controller, which processes the signal. For ease of understanding, our example will be a heating application. The set point (the desired control point) in this case is 68 degrees with a temperature differential of 2 degrees for the controller. When the sensor’s signal to the controller reports a temperature of less than the controller’s set point, the controller sends a signal to the final control device (hot water valve) to position to fully open until set point is achieved. When the controller receives a signal from the sensor that the set point has been achieved, the controller then sends a signal to the valve to position to fully closed.  The problem with On/Off control is over-shoot temperature of the desired system set point because of reaction time between sensor, controller, and final control device. Review: With On/Off, the controller asks “Is there an error?” The controller compares the actual value of the controlled medium to the set point through the sensor. As the controlled medium deviates from set point, the controller’s output cycles the final controlled device on, and when the set point is reached the controller’s output cycles the final control device off.

Floating control is a variation of On/Off control that requires a fast responding sensor and a slow-moving actuator connected to the final controlled device (valve or damper). Using the same example as the On/Off example above, when the sensed temperature drops below the set point of 68 degrees by the controlled medium’s sensor, the controller sends a signal to activate the actuator on the final control device. The actuator starts to slowly drive open the hot water valve, increasing the heat in the controlled medium. When set point is reached the actuator stops opening the final control device (hot water valve) and tries to hold at set point. If set point starts to be over-shot, the controller sends a signal to the actuator to start to drive close the valve. Review: Set point control is achieved when the sensor signal (from the controlled medium) starts to deviate from the controller set point. The controller sends a signal to the actuator of the final control device (valve or damper) to slowly drive open. As the set point is approached the controller sends a signal to the actuator, then the actuator stops and tries to maintain set point.  If set point is passed the controller sends signal to the actuator to drive the final control device to a closed position.

Modulating/Proportional represents the higher end of control positioning. In modulating/proportional control the output varies continuously and is not limited to being fully open or fully closed. Proportional means that the size of the output is related to the size of the error detected by the controller. The key phrase for modulating/proportional control is “Continuous Control Action.” The sensor, controller, and final control device act as one unit to maintain constant precise control over the controlled medium. Continuing with the previous example, when a modulating system senses a deviation from the set point of 68 degrees, the controller calculates the amount of the error (1 degree less than set point) and sends a signal to the actuator, which will drive open the final control device (valve or damper) by a certain percentage of the controlled medium’s set point deviation (1/2 degree) to maintain set point without over-shoot. The controller calculates how much the final control device needs to open without over-shoot and will start reversing the actuator to close the final control device to a percentage of the closed position to maintain set point.
Popular modulating control signals include 4-20 ma and 0-10 volts. If you were to look into a control panel like a Hoffman Enclosure you might see controls like a Honeywell UDC3200 that could be taking a 4-20 ma signal from a device like a Hawkeye 908 current transmitter and based on the control input signal from the Hawkeye 908 ( which would most likely be a 4-20 ma signal) the UDC 3200 controller would respond with a 4-20 ma output signal to a device like a Honeywell Variable Frequency Drive which would control either a fan or a pump. This is an example of how a proportional signal like a 4-20ma signal is used in modern HVAC controls.
If you are in Georgia or Florida,the control pros at Stromquist & Company can answer your control questions.

ASCO Control Panel for Gas Service

The ASCO relay control panels are designed to operate ASCO 120 volt and DC solenoid valves controlling gas flow to school kitchens, domestic cooking classes, metal shops, and laboratories. Many situations now call for a way to lock out gas supplies during closed hours.

Operating the key switch on the 108D10C (DC panel) or the 108D90C (120 AC panel), the control station energizes the relay to open a normally closed ASCO gas solenoid which turns on the gas flow.

Other Features:

*If the control voltage is lost completely or is reduced to approximately 50% of normal value, the relay de-energizes the normally closed valve to shut off gas flow.

*The gas valve will not re-open at restoration of power until an authorized person operates the key switch on the control station. The gas may also be shut off by depressing the normally closed pushbutton switch located on the control station.

*Shallow NEMA 1 flush-mounted enclosure.

*Clearly marked terminals and installation drawing on inside panel cover.

*Optional auxiliary pushbuttons 173A19 and 173A20 may be located at various accessible locations throughout the building.

*Explosion proof gas valves from ½” to 2” available.

If you are a Stromquist customer located in Georgia or Florida  and you need help sizing or ordering an ASCO Gas Control Panel you can contact Stromquist and Company at  www.stromquist.com or call us at               1-800-241-9471         1-800-241-9471. All others can order this product from one of our affiliates at CGNA.