To calculate energy usage and loads for a heating system, the question is:
Which side of the heating system should we measure to provide the most accurate and encompassing measurement results?
The simple answer would be the steam side since this is the heating medium providing the energy to the point of use location. However, when you start to look at the measurement technologies available and the difficulties presented in measuring a compressible medium it becomes questionable if this side is, in fact, the most appropriate measurement location.
Let me explain…
Due to the nature of the process (Steam) you are faced with a very hostile medium with extremely high temperatures. You’re also working with compressibility/density changes due to pressure/load variances and operating ranges that have extreme variations depending on the time of the year/season. Combined with available measurement technologies, you are very limited in a reliable, accurate measurement solution.
As a general rule, the most accurate and reliable technology available for this application is Vortex shedding. Vortex shedding uses a blunt body (called a shedder bar) inserted into the flow stream to create vortices as the flow passes over/around the shedder bar. Once vortices are shed, a pickup or sensing technology detects the vortices and the rate at which they are sensed is proportional to velocity.
Depending on the sensing technology employed, any particular vortex meter will have a maximum and minimum velocity range. These ranges will vary with the technology, but as a rule of thumb, Vortex meters are very linear and repeatable to +/-1.0% of rate or reading with a typical ideal meter turndown range of 20:1. Also, keep in mind that at the meter’s low velocity point—which is Reynolds number dependent on where the medium (steam) goes from transitional to laminar flow— the shedder bar no longer sheds vortices. This means the meter has nothing to measure and, therefore, goes to zero. That is, unless a fixed output is configured into the output circuit (which is more common than not by some manufactures) to make the meter read some value. To make the problem worse, depending on how the meter was initially sized for the application (oversized in most cases), 15% to 50+% of the actual load or flow could still be passing by the meter while the meter has no vortices to measure!
The first thought that comes to mind is a logical question:
Why would someone select an over sized meter knowing it has a finite low flow cut-off?
Well, it is not that simple. In order to properly size a vortex meter you need to know the maximum flow rate or load expected. That seems simple enough. Wouldn’t that just be the line size of the piping of where it will be installed? Unfortunately, this is not the case. In fact, in more than 90% of the installations where the maximum load can be determined, the actual line size is 1-3 pipe diameters too large or oversized for that load.
Another question that comes to mind is:
Why would someone overdesign a piping system that much?
Depending on the growth expectations for the point of use location, the line size that is supplying the line may have been selected for future loads. In addition, engineers like to use safety factors in order to prevent system under sizing, and in many cases these factors alone increase the line size an additional pipe diameter.
So, now you see why the variance (15%-50+%) in the load is still passing by the meter below the low flow cutoff. Keep in mind that the 15% is an ideally sized meter with the load and line size matching up with the meter maximum load capability and letting the full 20:1 turndown capability reach the ideal low flow cutoff point before going to zero output/reading.
So, let us recap the factors involved in measuring energy usage from the Steam supply side of the system:
1) Ideally, it makes sense to measure since Steam is the heating medium at the point of use.
2) Due to the nature of Steam (Gas) it is a very difficult medium to measure with high reliability, accuracy, and turndown.
3) Heating loads can, and do, vary widely with seasonal requirements, making the flow range extremely large.
4) The most well suited measurement technology is Vortex due to it’s high accuracy, linearity, and reliability under the demanding process conditions.
5) Limitations of the measurement technology (Vortex) are the operational rangeability or turndown, which is velocity dependent.
6) Measurement technology (Vortex) is not zero-flow-based. Has a low flow cutoff where meter goes to zero even though flow is still passing through the meter.
7) 15%-50+% of actual steam flows or load is unmeasured due to sizing and technology limitations of the flow meter.
So, if the Steam measurement has such limitations, we should start to investigate the Condensate measurement as an option…
First, how does condensate relate to steam with regard to energy or load? First let’s start by defining that a pound of Steam is equal or the same as a Lb of condensate. Obviously the volume of space they consume is different, but the Mass, pound for pound, is the same.
With that in mind, if we look at the fluid characteristics of Condensate, we now have a medium with much better properties, allowing a flow device to provide a measurement with reliability, accuracy, and turndown. Without having to worry about excessive temperatures and compressibility/density changes, the measurement becomes simplified and available to a wider range of technologies. However, that is not to say there are not measuring issues with the application.
Since the loads vary widely with seasonal changes the flows and velocities also vary. In addition, since steam is such an ideal carrier of energy, the amount of condensate in volume is much smaller than the steam. This means that the volume of condensate produced as the steam gives up its energy is very small. Depending on the measuring application (Pumped or Gravity condensate), this could result in very low flow/velocities (Gravity). This means condensate line sizes are much smaller than steam lines and that any flow technology in consideration will be much smaller in size/diameter or have very high turndown or low velocity capabilities.
When addressing reliability, if at all possible, all mechanical volumetric flow meters should be eliminated. Even though the medium is much more measurement friendly it is still at an elevated temperature and contains small particulate carried over from the steam piping system. This particulate will foul or clog mechanical meters over time, reducing both turndown and accuracy. Eventually, these types of meters cease to function— clogged to a point where the friction interface seizes. This leaves technologies such as vortex, ultrasonic, pitot tubes (Differential pressure producers), and magnetic, all of which provide accurate and reliable measurement to some degree without any moving parts.
When addressing accuracy and turndown we can instantly eliminate Vortex and Pitot tubes (Differential producers). In the case of vortex meters, as discussed above, the low flow cutoff and non-zero-based flow cause the same issue for condensate as they do for steam. In the case of differential pressure producing devices, resolution and accuracy at low flow become an issue that is compounded further by the squared root extraction of the error required to convert DP (differential pressure) to a flow measurement.
This leaves us with Ultrasonic and Magnetic. Ultrasonic, by its measurement nature prefers higher velocities to make the time of flight measurement accurately. This limits the low end velocity to about 1.0 ft./s to maintain an accurate measurement. For pumped condensate, this would be acceptable, but definitely not for Gravity condensate, which can easily approach velocities down to 0.01 ft./sec.
Now we’re down to Magnetic flow meters, which in the past have been hampered with the misconception that the conductivity of the condensate is too low to make a measurement. First let’s consider the conductivity the meter requires (3-5 uS/cm) and value of the water being generated to steam in the boiler. At this particular location, the conductivity may be too low depending on the type of boiler and the quality/capability of the RO system being used to generate the boiler feed water (0.1-3 uS/cm). However, once the steam is generated and starts moving through the piping system, mineral particulate will begin to contaminate the steam. This continues and worsens as the steam condenses into liquid as most systems are vented to atmosphere increasing contamination with the introduction of oxygen and its oxidizing effect of all metal surfaces. So, by the time the condensate reaches the Magnetic flow meter, the conductivity is most likely in the (5-30 uS/cm) range, which is well within the measurement range of the technology.
In the past, Magnetic meters have been limited with some of the same turndown issues as ultrasonic. But, these meters are now available with much lower flow capabilities while still providing very accurate measurements. With turndown capabilities of 500:1 at 1.0% of rate or reading accuracy, the Magnetic flow meter has become the technology of choice for this measurement technology.
So let us recap the measuring energy usage of Steam supply side of the system.
1) One pound of Condensate is equal to or the same as one pound of Steam.
2) Due to the nature of Condensate (Liquid), it is a much easier medium to measure with high reliability, accuracy, and turndown.
3) Heating loads can, and do, vary widely with seasonal requirements, making the flow range extremely large.
4) The most well suited measurement technology is Magnetic due to its high accuracy, linearity, and reliability under the demanding low flow process conditions.
5) There are virtually zero limitations of the measurement technology (Magnetic) with the advances in the turndown capabilities.
6) Measurement technology (Magnetic) is zero-flow-based, meaning there is no flow cutoff where the meter goes to zero even though flow is still passing through it.
7) All condensate captured or recovered is measured due to the technologies high accuracy, linearity, and reliability under the demanding low flow process conditions.
In an effort to answer the question of which side to measure the energy or load more accurately, Steam or Condensate, we proved that what seemed like the most logical and direct approach (Steam) was just the opposite, and by far, the most difficult and inaccurate. Condensate, on the other hand, offers a much better medium and readily available measurement technologies to provide accurate, reliable, repeatable measurements for the varying load requirements of a typical steam heating system.
The bottom line is…
The potential for 15-50% of the load to be left unmeasured on the Steam side (due to limitations in measurement technology and meter sizing, with unknown loads and oversized piping systems) makes the Condensate side not only more accurate with extremely high turndowns and zero-flow-based measurement technologies, but also much more direct and fool proof when sizing meters for system conditions.
Call us to discuss your heating system today and let Cadillac Meter help you find the best possible energy measurement solution.