The multiphase desander pressure drop model has been detailed in previous articles. The relationship between flow and pressure drop is complex because of the compressibility of gas. The actual volume of gas – a function of gas flow, pressure, temperature, and gas properties – along with the actual volume of liquid determines the total actual fluid throughput. Each change in inlet pressure or temperature will lead to a subsequent change in actual flow – which results in a change in pressure drop.

An example of the relationship between gas void fraction (GVF), liquid flow, and pressure drop is illustrated below. The combined liquid flow (power law) and gas void fraction (polynomial law) fit provides the resulting pressure drop. For a fixed pressure drop increasing the GVF requires a decrease in liquid flow.

No simple turndown relationship exists as does with liquid flow. Due to the combined gas-liquid flow the turndown is stated in terms of pressure drop, instead of flow rate.

• Minimum ΔP: 3-5 psid: Increasing GVF decreases minimum ΔP (or minimum velocity to achieve proper vortex flow)
• Maximum ΔP: 25-75 psid: Increasing GVF decreases maximum ΔP (maximum velocity fixed to minimize insert wear)