Grab Sampling Points in SWAS Panels
Prosoft Engineering
Technical Compliance Specialist
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Introduction
A well-designed SWAS Panel (Steam and Water Analysis System) does more than continuously monitor steam and water chemistry through online analyzers. It can also provide carefully designed grab sampling points that allow operators and laboratory personnel to collect representative samples for independent laboratory analysis.
Grab sampling is particularly useful for validating online analyzer measurements, investigating abnormal chemistry conditions, performing laboratory testing and monitoring parameters that are not continuously measured.
In thermal power plants, combined cycle power plants, HRSG systems and industrial steam-water applications, the combination of Online SWAS Panel monitoring and properly designed grab sampling points provides a more complete approach to steam and water chemistry management.
The quality of a laboratory result depends heavily on the quality and representativeness of the sample collected. Therefore, grab sample points should be treated as an important part of SWAS engineering rather than simply adding a drain valve to the panel.
What Is Grab Sampling?
Grab sampling means collecting a discrete sample from a process stream at a particular point in time for subsequent testing.
Unlike an online analyzer, which continuously or periodically measures a parameter, a grab sample provides a snapshot of the process condition.
A grab sample may be sent to a laboratory for analysis of:
Conductivity
pH
Silica
Sodium
Chloride
Sulphate
Dissolved oxygen
Iron
Copper
Phosphate
Other chemistry parameters
The exact laboratory test list depends on the plant chemistry program.
Why Are Grab Sampling Points Important in a SWAS Panel?
Grab sampling points provide several important functions.
1. Online Analyzer Verification
Laboratory results can be compared with online analyzer readings.
This can help identify:
Analyzer drift
Calibration problems
Sensor problems
Sample-conditioning problems
Unexpected process changes
2. Investigation of Abnormal Readings
If an online analyzer suddenly indicates high conductivity, sodium or silica, a grab sample can provide an additional source of information.
3. Measurement of Additional Parameters
Not every chemistry parameter needs to be continuously measured.
Grab sampling allows laboratory personnel to analyze parameters that are not installed as online analyzers.
4. Commissioning
Grab samples are useful during SWAS commissioning to compare analyzer performance with independent laboratory measurements.
5. Troubleshooting
When an analyzer reading appears abnormal, a properly collected sample can help determine whether the problem is with the process or the analytical system.
Grab Sampling vs Online Analysis
FeatureGrab SamplingOnline SWASMeasurementDiscreteContinuous/periodicResult availabilityLaboratory dependentImmediate/localHistorical trendingLimited unless recordedNormally availableAnalyzer verificationExcellentPrimary measurementAdditional parametersFlexibleRequires installed analyzerResponse to sudden eventDelayedRapidLaboratory testingRequiredNot always requiredSample handlingOperator dependentAutomated conditioning
The two approaches should generally be viewed as complementary rather than competing systems.
Typical Grab Sampling Locations
Grab sample points should be selected based on the plant process and chemistry-monitoring requirements.
Typical SWAS sample locations may include:
Condensate
Feedwater
Deaerator outlet
Boiler drum water
Main steam
Reheat steam
Make-up water
HRSG water circuits
Condensate polishing outlet
Cooling-water-related streams where applicable
The final sample-point selection should be based on the plant's process design and chemistry program.
Grab Sampling in Thermal Power Plants
Thermal power plants use multiple steam and water circuits.
Potential sampling locations include:
Condensate
Useful for checking contamination and condenser-related problems.
Feedwater
Can be sampled to verify chemistry conditions before water enters the boiler/HRSG system.
Boiler Drum Water
Grab sampling may support laboratory analysis of selected boiler-water parameters.
Main Steam
Condensed steam samples may be collected for laboratory analysis of steam purity-related parameters.
Make-Up Water
Laboratory testing can verify the quality of water entering the cycle.
The sample point should be designed so that the collected sample represents the actual process stream.
Grab Sampling in HRSG Applications
HRSG systems can have multiple pressure levels and water-steam circuits.
Depending on the HRSG design, grab sample points can be considered for:
HP drum
IP drum
LP drum
Feedwater
Condensate
Main steam
Reheat steam
Make-up water
Where multiple pressure levels are involved, each sample point must be engineered for its actual pressure and temperature conditions.
Design of a Grab Sampling Point
A typical grab sample arrangement can include:
Process Connection → Isolation Valve → Pressure Reduction → Cooling → Sample Valve → Sample Container
The exact arrangement depends on the process conditions and the laboratory sampling requirements.
Important components may include:
Primary isolation valve
Secondary isolation valve
Pressure-reducing device
Sample cooler
Temperature indicator
Sample flow control
Sample valve
Drain
Sample connection
Protective enclosure where required
Pressure Reduction for Grab Sampling
High-pressure steam and water samples cannot normally be collected directly into a laboratory container.
Pressure must first be reduced to a safe and suitable condition.
Possible components include:
Pressure-reducing valve
Restrictor
Needle valve
Pressure regulator
Sample conditioning assembly
The pressure-reduction system should be selected according to:
Process pressure
Process temperature
Sample flow
Sample chemistry
Required sample condition
Safe sampling procedure
Improper pressure reduction can cause flashing, splashing, unstable flow or loss of volatile components.
Sample Cooling
High-temperature process samples must normally be cooled before collection.
A sample cooler can be installed upstream of the grab sample valve.
Typical arrangement:
High-Temperature Sample → Pressure Reduction → Sample Cooler → Temperature Monitoring → Grab Sample Valve
The cooler should be appropriately sized for:
Sample temperature
Sample pressure
Sample flow
Cooling-water temperature
Cooling-water flow
Required sample temperature
The sample should be cooled to a temperature suitable for safe collection and the analytical method being used.
Sample Flow Control
Flow control is important for both safety and sample representativeness.
A grab sampling point may incorporate:
Flow-control valve
Needle valve
Flowmeter
Restrictor
Flow indicator
The operator should be able to establish a stable sample flow before collecting the final sample.
Very high flow can create safety problems, while excessively low flow can increase sample transport time and allow the sample to interact with the sample line for too long.
Sample-Line Materials
Material selection is important because the sample itself can be contaminated by unsuitable wetted components.
Potential materials include:
SS 316/316L
SS 304/304L
PTFE
PFA
PVDF
Other application-approved materials
For low-level measurements such as sodium and silica, cleanliness and material compatibility can be particularly important.
The selected material should be compatible with:
Sample chemistry
Temperature
Pressure
Required analytical accuracy
Sampling method
Sample Valve Design
The sample valve should allow controlled collection without introducing unnecessary contamination.
A good sample valve arrangement should provide:
Smooth flow control
Reliable shutoff
Minimal dead volume
Suitable wetted materials
Easy cleaning
Safe operator access
Where high-purity samples are involved, dead legs and stagnant volumes should be minimized.
Sample Conditioning Before Collection
Sample conditioning is essential when the original process is at high temperature or pressure.
A typical sequence is:
Process → Isolation → Pressure Reduction → Cooling → Flow Control → Grab Sample
Additional components may be required depending on the application:
Filtration
Degassing
Pressure control
Temperature monitoring
Sample distribution
The objective is to provide a sample that is both safe to collect and representative of the process.
Grab Sampling for Online Analyzer Verification
One of the most valuable applications of grab sampling is checking an online analyzer.
For example, if an online conductivity analyzer indicates an unexpected increase:
Verify the online analyzer status.
Check sample flow.
Check sample temperature.
Inspect the sample line.
Collect a representative grab sample.
Send the sample for laboratory analysis.
Compare the laboratory result with the online measurement.
Investigate the cause of any significant difference.
This approach helps distinguish between an actual process change and an analyzer/sample-system problem.
Grab Sampling and Silica Analysis
Silica can be measured using both online analyzers and laboratory methods.
A grab sampling point can be useful for:
Online analyzer verification
Troubleshooting
Periodic laboratory checks
Investigating sudden changes
Because low-level silica measurement can be sensitive to contamination, sampling equipment should be kept clean and suitable for the analytical method.
Grab Sampling and Sodium Analysis
Sodium measurements can be particularly sensitive to contamination.
A grab sample point for sodium analysis should therefore be designed with attention to:
Wetted materials
Sample-line cleanliness
Sample container cleanliness
Sample flushing
Dead volume
Sampling procedure
The laboratory method and sample-container requirements should be established before finalizing the sampling arrangement.
Grab Sampling for Conductivity and pH
Conductivity and pH are commonly used for checking steam-water chemistry.
For reliable grab sampling:
Establish stable flow
Flush the sample path appropriately
Control sample temperature
Use clean sampling containers
Minimize atmospheric exposure where relevant
Analyze the sample promptly where required
Some measurements can change after collection because of temperature change, gas exchange or chemical reactions. Therefore, the laboratory sampling procedure should specify appropriate handling and analysis time.
Grab Sampling and Dissolved Oxygen
Dissolved oxygen sampling requires particular care because exposure to atmospheric oxygen can alter the sample.
For DO verification:
Minimize air ingress
Avoid unnecessary splashing
Maintain appropriate sample flow
Use suitable sampling equipment
Follow the applicable analytical procedure
Analyze promptly where required
A poorly collected sample may not represent the actual dissolved oxygen concentration in the process.
Grab Sampling in Multipoint SWAS Systems
A multi-point SWAS Panel may include several grab sample connections.
For example:
Condensate → Grab Sample
Feedwater → Grab Sample
HP Drum → Grab Sample
Main Steam → Grab Sample
Make-Up Water → Grab Sample
Each point should be clearly identified.
Recommended identification can include:
Sample point number
Process source
Stream description
Normal operating conditions
Sampling instructions
Safety information
Grab Sampling in an Online SWAS Panel
A modern Online SWAS Panel can combine continuous analyzers with dedicated grab-sampling arrangements.
A typical system can include:
Sample Extraction → Pressure Reduction → Cooler → Flow Control → Online Analyzer
with a separate branch:
Conditioned Sample → Grab Sample Valve → Laboratory Container
This arrangement allows the operator to collect a sample under controlled conditions while the online analyzer continues monitoring the stream.
Grab Sampling Safety
Safety is one of the most important aspects of grab sampling.
Steam and water sample lines may contain:
High pressure
High temperature
Corrosive chemicals
Chemical treatment agents
Potentially hazardous process constituents
Sampling points should therefore be designed with:
Safe operator access
Proper pressure reduction
Adequate cooling
Isolation valves
Controlled sample flow
Suitable drainage
Protective guarding where necessary
Appropriate warning labels
Personnel should follow the plant's approved sampling procedure and PPE requirements.
Drainage Arrangement
After collecting a sample, the conditioned sample must be safely discharged.
Drainage should be designed to prevent:
Operator exposure
Floor flooding
Cross-contamination
Back pressure
Unsafe discharge
The drain should be compatible with the sample chemistry and plant drainage system.
Where different sample streams are involved, cross-contamination through common drains should also be considered.
Sample Identification and Labelling
Each grab sampling point should have clear identification.
Recommended information includes:
Sample point ID
Sample source
Process stream
Pressure
Temperature
Sampling instructions
Safety warning
Destination/laboratory reference where applicable
Good labelling reduces the possibility of collecting or recording the wrong sample.
Grab Sampling Procedure
A generic sampling procedure may include:
Step 1 — Confirm Sample Point
Verify the correct process source and sample identification.
Step 2 — Check Safety
Confirm that pressure and temperature have been reduced to safe sampling conditions.
Step 3 — Inspect the System
Check valves, tubing, cooler, flow and drainage.
Step 4 — Establish Stable Flow
Adjust the sample flow according to the approved procedure.
Step 5 — Flush the Sampling Line
Flush sufficiently according to the plant/laboratory procedure.
Step 6 — Collect the Sample
Use the correct clean sample container.
Step 7 — Close the Sampling System
Restore the sampling point to its normal operating condition.
Step 8 — Label the Sample
Record sample point, date, time and other required information.
Step 9 — Laboratory Analysis
Send the sample for analysis according to the applicable laboratory procedure.
Step 10 — Compare Results
Where applicable, compare laboratory results with the online analyzer.
Grab Sampling and SWAS Panel Maintenance
Grab sampling arrangements should be included in SWAS Panel Maintenance programs.
Maintenance activities can include:
Inspecting sample valves
Checking sample tubing
Checking pressure regulators
Inspecting sample coolers
Checking flow-control devices
Cleaning sampling connections
Inspecting drains
Checking labels
Verifying leakage
Checking sample containers and accessories
Reviewing sampling procedures
Sampling points should also be periodically evaluated for contamination, blockage and deterioration.
Common Problems with Grab Sampling Points
Blocked Sample Line
A blocked line can result in:
No flow
Slow response
Non-representative sample
Possible causes include deposits, corrosion products or damaged components.
Excessive Sample Temperature
Possible causes include:
Insufficient cooling
Cooler fouling
Low cooling-water flow
Excessive sample flow
Contaminated Sample
Potential sources include:
Dirty tubing
Unsuitable sample container
Residual sample
Incorrect flushing
Dead volume
Unstable Flow
Potential causes include:
Pressure instability
Valve problems
Restrictor blockage
Incorrect adjustment
Difference Between Online and Laboratory Results
Possible causes include:
Analyzer calibration
Sample contamination
Different sampling locations
Sample transport delay
Sample handling
Temperature differences
Analyzer/sample-system problems
A difference should therefore be investigated systematically rather than assuming that either result is automatically correct.
Troubleshooting Sequence
When investigating a discrepancy between grab-sample and online results, use:
Process → Sample Point → Conditioning → Sampling Procedure → Laboratory Method → Sensor → Analyzer → Signal
This sequence helps separate process chemistry problems from sampling and analytical errors.
Material of Construction for SWAS Grab Sampling Points
For SWAS Panel India applications, MOC should be selected according to:
Sample chemistry
Pressure
Temperature
Cleanliness
Corrosion resistance
Required detection level
Laboratory analytical method
Common materials can include:
SS 316L
SS 304L
PTFE
PFA
PVDF
The correct material should be confirmed for the specific application.
Grab Sampling in SWAS Panel India Projects
For SWAS Panel India installations, grab sampling design should account for local site conditions and plant operating practices.
Consider:
Ambient temperature
Cooling-water conditions
Accessibility
Drainage
Operator safety
Spare availability
Sample transport to laboratory
Laboratory location
Maintenance practices
Area classification
If the laboratory is located far from the SWAS Panel, sample transport time and preservation requirements should be considered.
Documentation for Grab Sampling Systems
A professionally engineered SWAS system should provide documentation such as:
Sample-point list
P&ID
Sample-conditioning schematic
MOC schedule
Instrument list
Valve list
Sampling procedure
Calibration information
DCS/SCADA tag list
Maintenance instructions
Spare-parts list
This documentation helps operators and maintenance teams understand the complete system.
How to Select Grab Sampling Points
Before finalizing grab sampling locations, consider:
Process
What stream needs to be monitored?
What is the process pressure?
What is the process temperature?
Is the sample representative?
Analytical
Which parameters require laboratory verification?
Which parameters are continuously monitored?
What laboratory method will be used?
Safety
Can the sample be safely collected?
Is pressure adequately reduced?
Is temperature adequately controlled?
Is drainage available?
Maintenance
Can valves and coolers be accessed?
Can sample lines be flushed?
Are spare components available?
Integration
Can the grab sample point be integrated with the online sample-conditioning system?
Benefits of Grab Sampling in a SWAS Panel
Properly designed grab sampling points provide:
Independent verification of online analyzers
Additional laboratory testing capability
Better troubleshooting
Flexible chemical analysis
Support during commissioning
Improved chemistry diagnostics
Detection of analyzer drift
Verification after maintenance
Support for process investigations
The combination of grab sampling and online measurement gives plant chemistry teams a more complete view of steam and water quality.
Why Choose Prosoft Automation for SWAS Panel Solutions?
Prosoft Automation can provide application-oriented SWAS Panel solutions incorporating both online analysis and appropriate sample-conditioning arrangements.
Depending on the project, a system can include:
Steam and water sample extraction
Pressure reduction
Sample cooling
Flow control
Filtration
Online analyzers
Grab sampling points
pH measurement
Conductivity measurement
Cation conductivity
Dissolved oxygen
Silica
Sodium
Phosphate where applicable
PLC/HMI
DCS/SCADA integration
Alarm monitoring
Data logging
Drainage
The final configuration should be engineered according to the actual process conditions, chemistry program and customer requirements.
Grab Sampling Point Checklist
Before commissioning a SWAS grab sampling system, verify:
Correct sample location
Representative sample point
Primary isolation valve
Pressure reduction
Sample cooler
Temperature monitoring
Flow control
Sample valve
Suitable sample tubing
Correct MOC
Adequate drainage
Safe operator access
Clear sample-point identification
Sampling procedure
Correct sample containers
Laboratory method defined
Online analyzer comparison procedure
Maintenance procedure
Frequently Asked Questions
What is a grab sampling point in a SWAS Panel?
A grab sampling point is a controlled connection that allows an operator to collect a discrete steam or water sample for laboratory analysis.
Why is grab sampling required when online analyzers are installed?
Grab samples provide independent laboratory verification, support troubleshooting and allow measurement of parameters that may not have dedicated online analyzers.
Can grab sampling and online analysis use the same sample system?
Yes. A properly engineered system can provide both online analyzer measurement and a controlled grab-sampling branch.
Where should grab sampling points be installed?
Typical locations include condensate, feedwater, boiler/HRSG drum water, steam, make-up water and other critical streams identified by the plant chemistry program.
Is pressure reduction required for grab sampling?
For high-pressure steam and water samples, controlled pressure reduction is normally necessary before safe sample collection.
Is cooling required?
High-temperature samples normally require cooling before collection. The sample temperature should be suitable for safe handling and the applicable analytical method.
How often should grab sampling points be maintained?
They should be inspected as part of the overall SWAS Panel Maintenance program. The exact frequency depends on operating conditions and plant procedures.
Can grab samples be used to verify silica and sodium analyzers?
Yes. Properly collected laboratory samples can be useful for independent verification, provided the sampling and laboratory methods are appropriate for the required measurement level.
Can Prosoft Automation provide SWAS Panels with grab sampling arrangements?
Yes. Prosoft Automation can engineer SWAS systems with online analyzers and application-specific grab sampling arrangements for power plants, HRSGs and industrial steam-water applications.
Conclusion
Grab sampling points in SWAS Panels provide an important link between continuous online monitoring and laboratory-based steam and water chemistry analysis.
While an Online SWAS Panel provides immediate information about selected parameters, grab sampling provides an independent method for verification, troubleshooting and analysis of additional chemistry parameters.
The effectiveness of a grab sampling system depends on representative sample extraction, proper pressure reduction, cooling, stable flow, suitable materials, clean sampling equipment, safe collection procedures and correct laboratory handling.
Grab sampling arrangements should also be included in the overall SWAS Panel Maintenance program so that valves, tubing, coolers, flow controls and drainage remain reliable.
For thermal power plants, combined cycle plants, HRSG systems and industrial steam-water applications, combining online analysis with well-designed grab sampling points can provide a stronger and more reliable chemistry-monitoring strategy.
Need a SWAS Panel with Grab Sampling Points?
The correct sample-point configuration depends on your process conditions, sample locations, pressure, temperature, chemistry parameters and laboratory requirements.
Request a quotation for this analyzer and discuss your application requirements with Prosoft Automation.