Proper location of level-measurement nozzle

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DETAILED DESIGN of a vessel includes determining the proper locations for level gauge/transmitter nozzles.

Figure above shows what happens when a gauge glass is connected to a vessel containing a vapour and two liquid phases. Assume that equal amounts of a liquid with Sg = 1.0, e.g. water, and a liquid with Sg = 0.5, perhaps oil, gradually flow into the vessel. Assume further that the span of the gauge glass is four feet, beginning one foot from the bottom of the vessel.
As the level of the oil rises, it flows into the glass. As both liquids rise further, water begins to enter the bottom of the glass. This is the state shown in vessel A. Up to this point, the glass shows a true indication of the level of propane in the vessel. Once water enters the glass, the oil is cut off. A constant plug, one foot thick, floats on top of the water. Its level no longer bears any obvious relationship to the actual level in the vessel. This is state shown in vessel B. The only relationship between the vessel and the glass is that the hydrostatic pressure is the same for both at the point where the glass taps into the vessel. A gauge glass is really nothing more than a manometer.
Once the level of the oil rises above the upper tap, it flows into the glass and the two interface levels adjust to the same elevation, as shown in vessel C. The gauge will continue to read correctly as long as its lower tap is in the water and the upper tap is in propane. If either fluid is withdrawn so that the upper tap is in the vapour space, the glass will once again read falsely.
This same analysis applies to any type of level indication based on density. Remember that a DP transmitter only gives a single reading, i.e. differential pressure. Therefore only a single quantity can be inferred. If the instrument is affected by only two fluids, it can yield the correct interface level between the two. If there are more than two distinct phases within the span of the two taps, it will give a reading based on the average densities of all the fluids within its span.
Capacitance or nuclear level transmitters will give similar results in multiphase situations, based on the average dielectric or nuclear absorption constants, respectively.
Question:
How can the process controls engineer be assured that the level readings are meaningful if even a gauge glass can't be trusted?

1. Make the entire vessel out of glass. But, this isn't usually practical.
2. Every section of a gauge glass must have separate taps into the vessel so that each pair of taps has no "hidden" phase floating in between. Either that, or accept the fact that until the interface reaches its "normal" range, gauge glasses and transmitters will read falsely.
3. For proper location of externally mounted level measurement nozzles, ensure that at least one nozzle is located in the top liquid phase and at least one nozzle is located in the bottom liquid phase.

Reference:

1. http://www.driedger.ca/ce6_v&t/CE6_V&T.html

1. Chemicalprocessing.com , Best Practices for Level Measurement

Type of Liquid Level Sensor AND ITS APPLICATION

 
 Technorati Tags: Level,Instrument,Process Design

LEVEL MEASUREMENT, which is the detection of the phase split between vapor/liquid, liquid/ liquid, vapor/solid and even liquid/solid, is a key parameter in the operation and control of modern industrial processes. Failure to measure level reliably has resulted in some of the most serious industrial accidents, including those at the Buncefield, U.K., fuel storage depot and BP’s Texas City refinery.
Type of level measurement includes:

Hydrostatic
This continuous indirect method measures the pressure due to liquid level and density plus over-pressure. The sensor measures the difference between this pressure and a reference one, normally atmospheric; so, it’s not well suited for vacuum and pressure service. Instruments come in flanged-mount­ed or rod-insertion styles, the latter not being recom­mended for turbulent conditions. Typical accuracies claimed are ±0.2% of reading but this depends on process fluid properties and conditions.

Float displacer
Suitable for point or continuous applications, it measures the change in buoyancy via a torque tube, lever or servo arrangement. The continu­ous measuring range is set by the displacer length im­mersed in the tank’s external cage, which is preferable for noisy applications, or servo mechanism. The point method uses a float, with the range being limited by the length of the float arm.

Nucleonic
Good for point or continuous duties, this non-contact method, which is independent of fluid density and viscosity, measures the signal strength of a radioactive source beamed across a vessel and has typical ranges of 0.24 m to 3.36 m. Accura­cies generally claimed are ±2% of reading. It’s the preferred method for monitoring level in flash vessels and reboilers under all temperature and pressure conditions.

Radar
Applicable to point or continuous applications, it measures the travel time of an impulse reflected from the liquid surface. Interfer­ence echoes from tank internals, and agitators are suppressed and signals can be characterized to give liquid volume. The sensor doesn’t contact the liquid but is exposed to headspace conditions, which don’t affect the measurement. Reflectivity requires the liquid dielectric constant, εR, to be at least 1.4 (hydrocarbons are 1.9–4.0, organic solvents are 4.0–10 and conductive liquids are over 10). Adjusting the antenna and signal conditions allows tailoring to the particular process, with guided radar used for low εR and turbulent conditions. The method can handle custody transfer because of its claimed accuracy of ±0.5mm.

Capacitance
For point or continuous service, it suits liquids that can act as dielectrics. Sensitivity increases with the difference in dielectric constants, δεR, between the liquid and the vapor space or between the two liquids. Special designs, involving coated and twin probes, are used when δεR is under 1.0, conductivities exceed 100 ʮmho, or to overcome probe build-up effects, and when vessel material is non-conducting. Typical accuracies claimed are ±0.25% of span. However, fluid properties affect mea­surements, so the method isn’t suitable for changing conditions. Maximum conditions are 200°C at 100 bar and 400°C at 10 bar.

Ultrasonic.
Suitable for point or continuous use, it is based on the time-of-flight principle. A sensor emits and detects ultrasonic pulses that are reflected from the surface of the liquid. The method is non-invasive, with some types being non-contact, and isn’t affected by εR, conductivity, density or humidity. Maximum conditions are 150°C at 4 bar.
Load cells.
Appropriate for point and continuous applications, such devices, which can be based on strain gauge or piezoelectric technology, measure the weight of the process vessel plus contents. Individual load cell accuracy of 0.03% of full scale is achiev­able but overall performance depends on correct installation practices to exclude external forces due to associated piping and equipment. For vessels with jackets, agitation and complex piping, it’s difficult to obtain an acceptable accuracy. When the container can be totally isolated, as in final dispensing and filling applications, precision weighing can be achieved.

Tuning fork
This method can detect point liquid level but isn’t suitable for viscous and fouling applica­tions. Maximum conditions are 280°C at 100 bar.

Conductivity
Good for finding point level, it requires a liquid conductivity exceeding 0.1 ʮmho and frequently is used on utility and effluent pump control systems.
Typical compara­tive costs
From lowest to highest, are: conductivity - capacitance - tuning fork - hydrostatic - displacer - ultrasonic - load cell - radar - nucleonic.

APPLICATION CONSIDERATIONS

1. Selection also must consider both the process and its control.

1. Process. It’s essential to understand the physical property variations of the process fluids and the phase changes that may occur within the process during normal and abnormal conditions.

1. Boilers, flash vessels and distillation column bottoms involve boiling liquids, resulting in noisy levels. Displacers in external cages frequently are used on steam generators and flash vessels, provided the process fluids are of low viscosity and relatively clean.

1. Non-contact nucleonic method will prove most reliable for distillation column bottoms, where reproducibility is more important than absolute accuracy. While expensive, it can be more than justified given its value in providing stable column operation and in preventing reboiler fouling due to loss of level.

1. Avoid the use of impulse lines in level systems if the process pressure varies and there’s a tendency for solids’ formation due to freezing, precipitation or polymerization. Purging the lines with inert gas or process compatible fluids will have limited suc­cess and is high maintenance.

1. Nucleonic level detection provides a powerful tool to perform on-line process diagnostics. Typical applications include monitoring level profiles in tray towers, distribution in packed beds, locating level build-up and blockages in vessels, and general flow studies.

Float, lead and Lag

Introduction:
When it comes to project activity management, activity sequencing is one of the main tasks. Among many other parameters, float is one of the key concepts used in project scheduling.
Float can be used to facilitate the freedom for a particular task. Let's have a look at the float in detail.
Float:
When it comes to each activity in the project, there are four parameters for each related to the timelines. Those are defined as:

• Earliest start time (ES) - The earliest time, an activity can start once the previous dependent activities are over.
• Earliest finish time (EF) - This would be ES + activity duration.
• Latest finish time (LF) - The latest time an activity can finish without delaying the project.
• Latest start time (LS) - This would be LF - activity duration.
The float time for an activity is the time between the earliest (ES) and the latest (LS) start time or between the earliest (EF) and latest (LF) finish times. During the float time, an activity can be delayed without delaying the project finish date. In an illustration, this is how it looks:

Leads and Lags:
Leads and Lags are types of float. Let's take an example to understand this.
In project management, there are four types of dependencies:

• Finish to Start (FS) - Later task does not start until the previous task is finished
• Finish to Finish (FF) - Later task does not finish until the previous task is finished
• Start to Start (SS) - Later task does not start until the previous task starts
• Start to Finish (SF) - Later task does not finish before previous task starts
Take the scenario of building two identical walls of the same house using the same material. Let's say, building the first wall is task A and building the second one is task B. The engineer wants to delay task B for two days. This is due to the fact that the material used for both A and B are a new type, so the engineer wants to learn from A and then apply if there is anything to B. Therefore, the two tasks A and B have a SS relationship.
The time between the start dates of the two tasks can be defined as a lag (2 days in this case).

If the relationship between task A and B was Finish to Start (FS), then the 'lead' can be illustrated as:

Task B started prior to Task A with a 'lead.'
Conclusion
For a project manager, the concepts of float, lead and lag make a lot of meaning and sense. These aspects of tasks are important in order to calculate project timeline variations and eventually the project completion time.
For Process Engineer, the basic concepts of scheduling is vital in ensuring that all process design related activities is carried in a planned matter without delaying the schedule.