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What Is a Slip Stream? | Process Engineering Glossary
What Is a Slip Stream?
In piping engineering and process engineering, a slip stream is a small continuous flow of process fluid diverted from the main process line, passed through analyser equipment, a monitoring instrument, a conditioning system, or a pilot-scale test unit, and either returned to the main line or sent to drain. The slip stream allows continuous representative sampling of the main process stream without interrupting the primary flow or requiring periodic manual sampling. It is distinguished from a full bypass, which diverts all the flow around a piece of equipment, by its small fractional flow rate, typically a fraction of a percent to a few percent of the main stream. The slip stream is a permanent or semi-permanent process piping feature shown on the piping and instrumentation diagram with its own line number, valves, and instrumentation.
Applications of Slip Streams
Refinery Analyser Systems
Petroleum refineries operate hundreds of online analysers connected to slip streams from product pipelines, distillation column products, reactor effluents, and feed preparation systems. Gas chromatographs continuously monitor the composition of reformate, LPG, fuel gas, and naphtha streams. Flash point analysers monitor diesel and kerosene product quality. Sulphur analysers monitor the sulphur content of desulphurised products against specification. These analyser slip streams allow the refinery to continuously verify product quality, detect off-specification conditions immediately, and make real-time adjustments to operating conditions that keep the products within specification.
Natural Gas Quality Monitoring
Gas transmission pipelines use custody transfer quality analysers fed by slip streams from the main pipeline. Calorific value analysers measure the heating value of the gas for billing purposes. Dew point analysers confirm that the gas meets the moisture and hydrocarbon dew point specifications for pipeline entry. Sulphur content analysers verify compliance with gas quality regulations. These slip stream systems operate continuously for months between maintenance intervals, making the reliability of the sample conditioning system as important as the accuracy of the analyser itself.
Pharmaceutical Water Systems
Pharmaceutical water systems for purified water and water for injection supply slip streams to conductivity, total organic carbon, and microbial monitoring instruments. The online instruments provide continuous assurance that the water quality meets pharmacopoeia specifications between periodic laboratory tests. The slip stream from the pharmaceutical water loop must be taken at a representative point in the recirculating loop, conditioned to the instrument’s operating requirements, and typically disposed of to drain rather than returned to the loop to avoid any possibility of contamination from the sample conditioning system.
Benefits of Slip Stream Systems
Continuous Representative Analysis
A slip stream delivers a continuous, uninterrupted supply of process fluid to the analyser, enabling continuous monitoring rather than the intermittent snapshot provided by manual sampling. Continuous monitoring detects process changes and upsets in real time, allowing the control system to respond before a product quality deviation has propagated through the entire process.
Analyser Protection
The sample conditioning system on a slip stream protects sensitive and expensive analyser equipment from the full temperature, pressure, and contamination of the process stream. By the time the process fluid reaches the analyser inlet, it has been conditioned to the temperature, pressure, particle loading, and phase condition that the instrument requires. This conditioning extends analyser service life and reduces maintenance intervals compared to instruments exposed directly to the unconditioned process stream.
Main Line Independence
The slip stream operates independently from the main process flow. Cleaning, recalibrating, or replacing the analyser requires only isolating the slip stream isolation valve, not shutting down the main process line. This independence makes slip stream systems highly practical for continuous process plants where the main line cannot be taken offline for instrument maintenance.
Limitations to Consider
Transport Delay
The time for the process fluid to travel from the extraction point through the sample conditioning system to the analyser adds a delay between a change in the main process composition and the analyser detecting it. For a slow-moving process where composition changes over hours, this delay is irrelevant. For a fast-moving process where composition changes over minutes, the transport delay must be minimised by shortening the sample line and increasing the sample flow velocity. The transport delay limits the speed at which the analyser output can be used for closed-loop control of the main process.
Representativeness Uncertainty
A slip stream can only be representative if the main process fluid at the extraction point is well-mixed and homogeneous. In two-phase or multiphase streams, the extraction of a representative sample is challenging because different phases preferentially flow to different regions of the pipe cross-section. Poor extraction point design produces a slip stream that is systematically enriched or depleted in one phase relative to the bulk, giving the analyser a biased picture of the main stream composition.
Maintenance Demand
Sample conditioning systems accumulate scale, corrosion products, and biological deposits in their filters, regulators, and heat exchangers. These deposits progressively degrade the sample quality and the analyser performance if the conditioning system is not maintained on a regular schedule. For continuous process plants that rely on the analyser output for product quality control, the sample conditioning system maintenance is as critical as the analyser calibration itself.
Slip Stream FAQ
What is a slip stream in process engineering? A slip stream is a small continuous flow of process fluid diverted from the main process line to supply an online analyser, monitoring instrument, or test system with a representative sample of the process fluid. Process engineering uses slip streams to enable continuous process monitoring without interrupting the main production flow. The slip stream passes through a sample conditioning system that adjusts the pressure, temperature, phase, and particle content to the requirements of the instrumentation before the fluid reaches the analyser. Both the slip stream extraction point and the return or drain connection appear on the piping and instrumentation diagram with dedicated line numbers and valve tags.
How does a slip stream support quality assurance and closed-loop control? Quality assurance in continuous manufacturing depends on reliable real-time composition monitoring, which slip stream analysers provide. The analyser output feeds directly into the closed-loop control system as a controlled process variable, allowing the control system to adjust operating conditions immediately when the analyser detects a product quality deviation. The strainer or filter in the slip stream sample conditioning system protects the analyser from particulate contamination that would produce erratic readings and bias the control action. In bioprocessing applications, slip stream analysers provide continuous fermentation performance data that the control system uses to adjust aeration, feed rate, and temperature to maintain the target dissolved oxygen and nutrient concentrations throughout the batch.
What are the key design requirements for a reliable slip stream system? A reliable slip stream requires an extraction point positioned where the main stream is well-mixed and representative, a sample flow rate high enough to minimise transport delay and flush the analyser between measurements, and a conditioning system matched to the specific analyser requirements for temperature, pressure, phase, and cleanliness. The isolation valves must allow the analyser and conditioning system to be isolated for maintenance without shutting down the main process line. For hazardous or toxic process fluids, the slip stream disposal system must handle the sample safely after the analyser, whether that means returning it to the main line at a lower-pressure point downstream or routing it to a closed drain system. The complete slip stream system from extraction tapping through conditioning to analyser and disposal must be documented on the piping and instrumentation diagram with sufficient detail for operators and maintenance personnel to operate and maintain it safely.
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