Isokinetic nozzles for steam sampling
The proper design of a sampling system is critical for a sample that is representative of the stream to be measured.
ASTHA MAHENDRU, MEGHNA BAHL and PARTHA MONDAL
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Refineries around the world generate high pressure steam for process applications and power generation. Poor water and steam chemistry can lead to corrosion, scaling and other problems, which can cost a plant millions of dollars.
In steam generating plant, the impurities in steam are controlled in parts per billion (ppb). Impurities in the steam may be derived from boiler water carryover, inefficient steam separators, natural salt solubility, and leakage through water cooled condensers, contaminated condensate rerun and many other factors. Impurities can cause hydrogen damage, corrosion fatigue, scaling caused by hardness compounds or silicates and carryover and fouling of super-heaters, re-boilers and turbines, and so on. This may lead to unwanted down time or preventive maintenance.
To avoid the damage or deterioration of equipment like boilers and turbines, it is necessary to measure accurately the purity of steam. Sample withdraw, transport, collection and handling are often major sources of error that can lead to incorrect or unnecessary corrective actions. The correct technique for sample collection or extraction plays a vital role in accurate measurement of steam.
This article explains design concepts and procedures for extraction and transportation of samples of saturated and superheated steam, and selection and application of the extraction nozzle.
Principle of isokinetic nozzles
An isokinetic nozzle ensures that all phases of the sampled fluid enter the sampling nozzle with the same velocity and direction as that of the sampled stream velocity.
Isokinetic sampling is necessary as the sampled stream is almost always a two-phase fluid and the second phase has a very different chemical composition than the steam or water. In addition, the second phase (droplets or particles) has a density and inertia that is different from the primary phase (gas or liquid) and therefore would not be proportionally represented in a sample that is not withdrawn isokinetically.
Minimum requirements for isokinetic conditions to prevail are:
• No divergence of flow lines around the nozzle inlet
• The velocity into the nozzle should be the same as the velocity in the pipe in which the fluid is sampled, at the tip location
• The nozzle must face directly into the sampled stream in order to minimise the divergence of the flow lines at the nozzle inlet.
Types of isokinetic sampling nozzles
Single port nozzles are generally located at a known distance from the pipe wall where fully developed turbulent flow exists, which enables the determination of velocity at any location in the stream line. These nozzles are easy to install and available at a reasonable cost.
Single port nozzles are most frequently located at a distance from the pipe wall where the actual velocity equals the average velocity, typically 0.2 R of the pipe.
Multiport nozzles are used to sample at various locations across the cross-section of the pipe. These nozzles may be used only at locations where the velocity profile across the pipe can be determined.
These nozzles (probes) are rarely used because of their complexity, cost, non-isokinetic characteristics and problems with use in large pipes.
In an ideal situation, isokinetic nozzles are installed in vertical pipes, preferably with the process flow in the downward direction. Under these conditions, gravity will have a minimal effect on the distribution of the suspended phase. If a long vertical section is not available, the nozzle may be installed in a long horizontal section, provided it is installed on the top of the pipe between the 10 and 2 o’clock positions to keep the nozzle dry during inactive periods. The nozzle opening should face upstream.
If the velocity of the sample at the probe entrance is different from the velocity of the sample stream, momentum effects will result in a non-representative sample.
When the sampling velocity is too low, that is the velocity in the sample tube is less than the velocity in the main stream, the particles in the stream will enter the sampling tube due to greater inertia while the gas is diverted around the tube. This gives a higher particulate concentration in the sample tube than in the main stream (see Figure 1).
Similarly, if the sampling velocity is too high, that is the velocity in the sample tube is greater than the velocity in the main stream, the inertia of the particles will keep the particles from following the flow lines, which converge into the sampling tube. This gives a lower concentration in the sampling tube than in the main stream (see Figure 2).
Sampling nozzles should be adequately supported and should be designed to prevent failure due to flow-induced vibration, thermal stress cycling and other possible causes.
For superheated steam applications, it is not recommended to install the nozzle in locations where the steam temperature is not at least 100°F above the saturation temperature.
It is not recommended to install the nozzle immediately after desuperheaters or in locations where there are large temperature changes, or where there is high carry-over of sodium hydroxide. Where these conditions exist, there should be more frequent inspection of the nozzle, nozzle attachment, valves, and welded tubing up to the primary cooler. If installed downstream of desuperheating sprays, the nozzle’s location should be far enough downstream where complete mixing has occurred.
Transportation of sample
The sample lines should be as short as possible, sloping downwards along the entire length. The bore size should be as small as possible to facilitate flushing, minimise conditioning requirements, reduce lag times and changes in the sample composition, and provide adequate velocity and turbulence. However, care should be taken in design as small tubing is vulnerable to mechanical damage.
The routing of the sample tubes should be in such a way as to protect them from exposure to extreme temperatures. Also expansion loops should be provided to prevent undue buckling and bending due to large temperature changes.
The isolation valves on the nozzle and the sample tubing between the isolation valves and the primary sample cooler should have approximately the same bore as the nozzle. This will maintain a constant flow velocity and minimise the amount of deposition between the nozzle and the primary cooler by eliminating dramatic pressure changes.
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