Since the beginning of the industrial revolution, U.S. manufacturers viewed steam as an inexpensive commodity. Affordable and abundant energy resources made the cost of producing steam an afterthought. During this era, the concept of monitoring steam production and delivery wasn’t top of mind. Instead, if the plant needed more steam, capacity could easily be increased by adding another boiler.
This operational model remained practical for more than 100 years. It wasn’t until the oil embargo of the 1970s that U.S. manufacturers became aware of the cost associated with generating steam. To ensure each pound of steam was being used efficiently, manufacturers started to monitor steam production and distribution through their process lines.
However, toward the end of the 1970s, the cost of energy leveled off and started to decline during the 1980s. It wasn’t until 1990 that manufacturers again saw energy prices increase and the cost to produce steam rise again. This time, energy prices continued to increase through the decade and into the present day.
In addition to progressively rising energy costs, regulatory action by the U.S. government required manufacturers to report how much pressure was being delivered through their process lines and at what temperature. This especially was the case for manufacturers in the pharmaceutical and food process industries who used steam to sterilize production equipment.
View more boiler content on PlantServices.com
Measuring steam output
The traditional method of measuring steam’s velocity, mass flow rate and physical properties directly from the boiler is effective for verifying the output and temperature of steam, but it has a limitation. It doesn’t account for downstream leakage. If steam escapes as it travels through the distribution lines, it won’t reach the point of intended use. To drive greater efficiency along the distribution process, verify the steam’s quality as it enters the production area.
The best way to measure steam en route to its destination is to install a second measurement point. If points along the process line are thought to be potential sources of leakage, you can install additional temperature and pressure instrumentation to continuously monitor for possible leaks, and then make the necessary repairs.
These additional sensors foster better management by monitoring the variables that affect steam efficiencies. Knowing the steam flow rate and pressure lets you determine the viability of the process line. Specifically, any leakage in the line normally results in the loss of pressure and flow.
Monitoring temperature allows you to monitor the effectiveness of the insulation. A temperature drop, along with a decrease in flow rate or pressure, suggests leakage. Temperature loss without a noticeable change in flow rate can be an indicator that there are gaps in or insufficient layers of insulation surrounding the pipe. Additionally, monitoring multiple points along the process line ensures regulatory requirements are met in regards to reporting steam pressure and temperature.
The financial aspect
Losing steam as it travels through a line affects operational costs. If it costs $5 to produce 1,000 lbs of steam at 100 psi, a half-inch downstream leak wastes 835,000 pounds of steam per month, equivalent to $4,175 per month ($50,100 annually).
To look at this another way, lb of 100-psi steam contains about 1,200 BTUs. If the steam is produced at 85% efficiency, the required input energy is 1,411 BTUs per pound. Therefore, producing 1,000 pounds of steam requires a least 1.4 million BTUs. One MCF of natural gas contains 1 million BTUs. This means that if natural gas cost $7 per MCF (Armstrong Intl.), 1,000 lb of steam will cost $9.80.
Tools for steam measurement
The orifice plate is the traditional tool for measuring steam velocity and mass flow rate. The instrument is widely used in industry and has been for more than 50 years. However, the instrument’s design makes it susceptible to wear and is optimized for only a single condition. Additionally, the orifice plate alone isn’t able to convert flow to a fixed rate measurement. Instead, pressure drop has to be converted to flow by means of a differential-pressure measurement device.
Under normal operating conditions, an orifice plate requires a nominal 5 psi of pressure drop to function. Upwards of 60% of the pressure drop across an orifice plate is unrecoverable, contributing to the overall cost to operate the steam measurement system. Newer meters, such as the vortex meter, operate with less than 1 psi of pressure drop — a substantial benefit when accounting for the cost of producing steam.
Modern steam meters also provide additional benefits, such as converting velocity into mass flow (such as kilograms or pounds per unit time). By converting the flow measurement into mass flow automatically, a manufacturer is able to more easily place additional meters downstream to monitor for leakage. For instance, if steam is losing heat as it travels through the process line, monitoring volumetric flow rate will be ineffective. Changes in the temperature and pressure affect the volumetric flow rate. This means that, for example, 1,000 cubic feet measured off the boiler at 25 psi isn’t equivalent to the same reading measured downstream, where the pressure is now only 20 psi.
The ability to compensate for temperature and pressure and to convert the output to mass flow provides a measurement that can be effectively compared, regardless of changes to the operating conditions at the points of measurement.
Additionally, new metering technologies are more resistant to wear and are capable of measuring a wider range of flow rates. For instance, an orifice plate designed for a maximum flow rate of 100 cf/min might be able to measure only 15 cf/min at the low end of its range. Other technologies, such as the vortex meter, have a wider turndown and might be able to measure flowrates, in this example, as low as 5 cf/min.
Future of steam management
The cost of energy in the United States is predicated to rise as the global demand for natural resources continues to increase. Add to this the pending U.S. legislation that might require U.S. manufacturers to measure and control their carbon dioxide emissions under a cap-and-trade system, and the cost of producing steam could soar. To manage rising energy costs, manufacturers will need to ensure that production processes are lean and monitored affectively.
The best technique for improving the efficiency of steam production and delivery is to have a minimum of two monitoring points (departure and arrival) on the distribution line. If leakage is possible at junctions along the distribution line, consider installing additional sensors. The ability to measure steam quality effectively through the process system is crucial for controlling costs and meeting regulatory reporting requirements.
Jack Roushey is global products marketing manager for flow and level products at Honeywell Process Solutions, Fort Washington, PA. Contact him at firstname.lastname@example.org and (215) 641-3029.