A well-designed steam and water sampling system condition samples for the precise measurement of various chemical and physical properties for both grab samples and for on-line analysis.
In other words, a properly designed steam and water sampling system preserves the properties of the sample so that analysers (on-line or laboratory) can provide the most reliable measurements possible.
Sample systems must be able to handle a wide variety of sample temperatures and pressures for both water and steam. The three critical variables to control are temperature, pressure and velocity.
The following discussion describes a typical, well designed sample system suggested by Sentry Equipment Corp* and recommended by both ASTM and EPRI.
V-1 Inlet Isolation Valve
The sample isolation valve is used to isolate the sample from the system for maintenance and safety and should be appropriately rated for the pressure and temperature conditions of the sample.
V-2A Blow Down Valve
The blow down valve is also rated for the sample conditions and is used to temporarily re-direct the sample flow at high velocity to a blow down header to bypass the sample conditioning system and purge contaminants that can accumulate in the sample lines. This valve is particularly important for systems such as combined cycle plants that change temperature and pressure often, generating large amounts of particulate.
SC-1 Primary Sample Cooler
The primary sample cooler is used to cool the steam or water sample to within 5° F (2.8° C) of the cooling water supply temperature. The primary sample cooler should be a high efficiency type with dual helical coil with dual baffles and as a minimum be constructed of 316/316L tubing and 304 SS shell for most applications.
The sample cooler should have the capacity to cool a liquid sample flow rate of about 1200 cc/min for ¼" O.D. tubing systems and up to 3000 cc/min for 3/8" O.D. tubing systems and somewhat less for steam samples.
VREL Variable Pressure Reducing Element (Valve)
The Variable Pressure Reducing Element effectively reduces pressure and helps to adjust sample flow for samples with pressures above 500 psig (34 bar). The VREL reduces pressure over a large surface area, has a wide adjustment range (56 turns from full open to closed) and is cleanable in place. These features offer better sample flow control, ease of maintenance, and a longer service life than other pressure reducing devices.
For pressures below 500 psig (34 bar) a needle valve rated for the service is acceptable for reducing sample pressure and controlling flow.
V-3 Low Pressure Blow Down Valve
three-way valve may be used to periodically flush the sample line, sample cooler and VREL during normal operation by temporarily bypassing the balance of the sample conditioning system, analysers and grab sample.
Note: Typically only one blow down option is used; either the high pressure blow down (V-2) or low pressure blow down (V-3).
TSV-1 Thermal Shut Off Valve
A resettable, mechanical, thermal shut off valve (TSV) is used to protect operators and analysers from hot samples in the event that the cooler is run at over-capacity due to sufficient cooling water or too much sample flow, or a scaled cooler.
The TSV is usually selected to trip at temperatures at or above about 49 °C (120 °F) since this is just below the typical maximum operating temperature of 50 °C (122 °F) for most instruments.
SC-2 Secondary Sample Cooler
EPRI recommends the use of secondary sample coolers along with a closely controlled chilled water supply to provide a sample temperature of 77 °F +/- 1 °F (25 °C +/- .5 °C).
Most analyser makes offer temperature compensation for most measurements; however, these algorithms have limits. By controlling sample temperature to within 1 degree of 77 °F +/- 1 °F (25 °C +/- .5 °C) which is the standard temperature reference for water analysis, temperature is no longer a variable.
Sample systems with secondary coolers and a Temperature Control Unit (TCU) offer more temperature stability which in turn provides a more consistent analysis baseline for setting action limits. Customer’s with widely variable coolant temperatures for primary coolers find the use of a secondary cooling system invaluable.
FI Flow Indicator
A rotameter or flow indicator is used to confirm that the sample flow rate is set to the desired velocity of 6 ft/sec (1.8 m/sec). A flow rate of 1200 cc/min in ¼” O.D. tubing produces a velocity of 6 ft/sec (1.8 m/sec). This velocity sets up an equilibrium condition in which the constituents in the sample to be measured are properly transported to the analysers or grab sample.
If the velocity is higher than this value, crud bursts comprised of materials collected on tubing walls are sent to the analysers. If the velocity is lower than the desired value, material tends to stick to the tubing walls instead of being measured. If the samples are not comprised of the actual constituents that are present in the system at the point of extraction, they are not representative.
PI Pressure Indicator
Measures and displays the pressure of the conditioned sample.
TI Temperature Indicator
Measures and displays the temperature of the conditioned sample.
BPRV Back Pressure Regulator/Relief Valve
The Back Pressure Regulating and Relief Valve combines the functions of a pressure regulator (back pressure regulator) and a relief valve.
The BPRV acts as a shock absorber to “absorb” changes in system pressure to maintain a fixed back pressure on the analyser branch lines. This helps to ensure that the analyser cells always receive the desired amount of flow for proper measurements.
The BPRV also has the capacity to relieve up to 5 litres per minute of sample flow to the sample sink or drain should there be a major system upset.
The discharge of the BPRV also serves as the grab sample point.
FICV Flow Indicating Control Valves
Flow indicating control valves are used to set and confirm the desired flow rates for each of the individual analyser measurements. Poorly designed sample systems rely solely on the FICVs to set total sample flow. The only way to account for the grab sample flow and set the desired total sample flow is to directly measure it with a total flow indicator (FI).
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