2000 Hook-up Book

Steam Meters

Selecting a Steam Meter Before selecting a steam meter it is important to understand how a meter’s performance is described. The overall performance of a meter is a combination of Accuracy, Repeatability and This is the measurement (expressed as a percentage) of how close the meter’s indication of flow is to the actual flow through the meter. There are two methods used to express accura cy (or percentage of uncertainty) and they have very different meanings. a.Measured Value or Actual Reading Example: Meter is ranged 0 1000 lb/h and has a specified accuracy of ± 3% of Actual Reading At an indicated flow rate of 1,000 lb/h, the true flow rate lies between 1,030 and 970 lb/h. At an indicated flow rate of 100 lb/h, the true flow rate lies between 103 and 97 lb/h. b.F.S.D. or Full Scale Deflection Example: Meter is ranged 0 1000 lb/h and has a specified accuracy of ± 3% FSD At an indicated flow rate of 1,000 lb/h, the true flow rate lies between 1,030 and 970 lb/h. At an indicated flow rate of 100 lb/h, the true flow rate lies between 130 and 70 lb/h (i.e. ± 30% of Reading !). Turndown. Accuracy

Repeatability This describes the ability of a meter to indicate the same value for an identical flowrate over and over again. It should not be con fused with accuracy i.e. the meter’s repeatability may be excel lent in that it shows the same value for an identical flowrate on several occasions, but the reading may be consistently wrong (or inaccurate). Repeatability is expressed as a percentage of either actual reading or FSD. Good repeatability is important for observing trends or for control e.g. batching. Turndown Sometimes called Turndown Ratio, Effective Range or even Rangeability. In simple terms, it is the range of flow rate over which the meter will work within the accuracy and repeatability toler ances given. If a meter works within a certain specified accura cy at a maximum flow of 1,000 lb/h and a minimum flow of 100 lb/h, then dividing the maximum by the minimum gives a turndown of 10:1. A wide turndown is partic ularly important when the flow being measured is over a wide range. This could be due to a vari ation in process e.g. a laundry could be operating 1 machine or 20 machines (20:1 turndown), or due to seasonal variations in ambient temperature if the steam is being used for space heating - the difference in demand between mid winter and mid summer can be considerable. Generally the bigger the turndown the better. The Spirax Sarco family of meters covers a wide range of sizes, turn down, accuracy, and repeatability as detailed in Table 18.

The steam meter is the basic tool that provides an operator or man ager, information vital in monitoring and maintaining high efficiency levels within a plant or building. This information can be split into four categories: Plant Efficiency • Is idle machinery switched off? • Is the plant loaded to capacity? • Is plant efficiency deteriorating over time indicating the need for cleaning, maintenance and replacement of worn parts? • When do demand levels peak and who are the major users? This information may lead to a change in production methods to even out steam usage and ease the peak load problems on boiler plant. Energy Efficiency • Is an energy saving scheme proving effective? • How does the usage and effi ciency of one piece of plant compare with another? Process Control • Is the optimum amount of steam being supplied to a cer tain process? • Is that steam at the correct pressure and temperature? Costing and Custody Transfer • How much steam is being sup plied to each customer? • How much steam is each department or building within an organization using?

SYSTEM DESIGN

Table 18: Specification of the Spirax Sarco Meter Range

Meter Type Orifice Plate

Sizes 1" -24"

Accuracy

Turndown

Repeatability

± 3% of Reading

4:1

± 0.3% of Reading ± 0.25% of Reading ± 0.25% of Reading ± 0.25% of Reading ± 0.5% of Reading

Gilflo (Made to Order) 2" - 16"

±1% FSD (± 1% of Reading with flow computer) ± 2% FSD (± 1% of Reading with flow computer) ± 2% FSD (± 1% of Reading with flow computer)

100:1 100:1 100:1

Gilflo (S.R.G.) Gilflo (I.L.V.A.)

2" - 8" 2" - 8"

Spiraflo

1-1/2" - 4" ± 2% of Reading (50% - 100% of meter range)

25-40:1

± 1% FSD (1% - 50% of meter range)

59