STREAM CONDITIONING - MIXING
DESIGN BASIS - MIXING IN A PUMPED FAST LOOP SAMPLING SYSTEM
Crude oil sampling according to API Chapter 8 and ISO 3171 implies the need for the creation of a homogeneous mixture of the process stream before the sample take-off point. The process mixing/conditioning ensures that samples are representative of the main stream. It is public knowledge that when the C1/C2 ratio is higher than 0.9 we have a stream that can be considered to be "well mixed". This C1/C2 ratio is derived from the “Rate of Energy Dissipation” induced into the process stream.
In EESIFLO’s proposal of a fast loop sampling system, the mixing concentration ratio is achieved with the combined “Rate of Energy Dissipation” of a bend in the piping layout, the blade elements (static mixer) and a pump powered sample-return nozzle.
Fig 10.1 EESIFLO Hybrid Fast Loop Sampling System
The Static Mixer is highly recommended for mixing simply because it is the most cost effective means for this application. Unfortunately, the performance of the static mixer can deteriorate considerably when the flow velocity falls below 0.6m/s. Common specifications require that sampling take place at all flow rates, even at low flow. A popular method of determining sample grab rate is “proportional to flow” sampling where the grab rate increases with velocity while inversely there are long dwell times at lowest flow. Since there are long dwell times (very few sample grabs) at lowest velocities, it could be argued by some that “grabbing” during those periods will have little effect on the overall composite sample while others do not want to miss out on a single drop at any time during any flow period. These systems are designed and built for the latter group.
Pump Circulating Loop Mixing is highly recognized as the best alternative for mixing at all flow rates. This is somewhat true, provided the pump is sized to deliver a substantial velocity at the outlet/nozzle i.e at a velocity that is much higher than the main stream. In many cases, the higher flow rates can become problematic (not just technically) but from an economical and practical standpoint. Engineers who have been left with no other choice but to use pumps and nozzles might want to consider other alternatives provided by EESIFLO.
EESIFLO has successfully designed and supplied oil companies a system that combines a static mixer with fast sampling fast loop pumps and nozzles. This hybrid method of stream conditioning meets the technical requirements at high and low flow. The logic dictates that at high flow rates there is sufficient energy for the static mixer to perform optimally and at a low flow rate the pump and nozzle system can be triggered to condition the media.
EESIMIX STATIC MIXER
A Static Mixer is a fixed arrangement of baffles enclosed in a tube or pipe. The process-stream flow is normally adequate in supplying all of the energy requirements for complete mixing. Since there are no moving parts, there are literally no maintenance requirements which can be considered a huge benefit to users who have been struggling with motorized equipment. As long as flow exists, the fluid can be mixed on a constant basis.
The EESIMIX mixer is a triple-action motionless mixer, carefully designed to contain pressure drop to a mimimum.
EESMIX Static Mixers for fast, efficient, low energy in-line mixing
The following graph shows the pressure drop across the span of flow rate with this specific design.
Frequently Asked Questions 1: Will the Static Mixer work at low flow?
Static Mixers are generally designed for flow velocity between 0.6 m/s to 4m/s. Given special attention to the angle adjustment and arrangement to the baffles the static mixer is able to achieve mixing effect at low flow without compromising the pressure drop at high flow (within 4 psi). Most automatic sampling systems are proportional to flow but there is a concern with some that the static mixer does not fulfill the mixing specifications while others deem this to be insignificant. The graph illustration below explains a typical proportional to flow rate sampling scenario. At the minimum flow of a process it is evident the sampler sample collection interval is much longer than it is at maximum flow.
Certain groups have determined that this has minimum influence on the overall composite sample i.e. the proportion of time for low flow regimes is much less than other times and might be considered insignificant while others prefer to maintain mixing for sample collection at all times, even during the minimal low flow. In both cases EESIFLO has a solution by either fabricating crude oil sampling systems with static mixers or a hybrid combination of pumps/nozzles/mixers where smaller pumps are installed that only need to be triggered during low flow periods.
The above answers apply only to pipe size above 2 inches.
EESIFLO is a company that can design a more economical complete sampling system with capital equipment savings and energy conservation in mind and still meet user requirements.