The following information may be useful for your sudies

As stated above, efficient and reliable condensation of steam is directly dependent on steam velocity. By using a steam pressure reducing valve for control, steam flow will become sub-sonic. The way to overcome this problem is to inject steam at choked flow conditions. Choked flow conditions allow steam to be injected at sonic flow velocities.

ProSonix’s unique method of steam injection utilizes an internal steam control to precisely deliver the appropriate mass flow of steam, and not the pressure, for the required heating. This is achieved via and integral Pneumatic Actuator, and a variable position stem plug in the steam jet diffuser. They do not throttle or regulate steam pressure. This design offers a precise method of steam control through a choked flow control delivery of the steam.

Choked flow is the phenomenon of accelerating a vapor to maximum velocity by creating a pressure differential through an engineered nozzle. By establishing choked flow, the steam mass flow can be metered to precisely control the heating of the liquid. This produces predictable results based on position of the stem plug. Through a variable area steam diffuser, steam flow is metered at the point where steam and liquid first contact and mix. Internally Modulated DSI heating controls the mass flow of the steam and not the pressure.

Key Benefits of Internally Modulated Steam:

No Process Upsets - High velocity steam flow optimizes the steam mixing and condensation with the liquid and eliminates problems with vibration/steam hammer.

No Steam Control Valve (PRV) Required - This method eliminates the need for an external steam control valve or downstream mechanical mixing devices.

Reliable Temperature Control – Rapid and complete condensation of the steam allows for temperature reliable control of +/- 1 °F. Self Cleaning Design – High velocity steam also is self cleaning and eliminates debris along scale& mineral build-up on the steam diffuser.

Lower Maintenance Costs – Proper condensation of the steam eliminates excessive wear.

Low Pressure Drop – Typically 1-2 psig reduces pump demand and energy consumption.

REF:www.pro-sonix.com/HeatingprocesswaterInfo.asp?...Inline+Water+Heati.

At least you recognize that there is a problem. Most people can't appreciate the problems with measuring two-phase flow--even when you explain it to them in great detail. If it's dry (superheated) steam, just measure the water and steam separately before mixing and add the two. A high quality nozzle is best for the water and a sharp-faced orifice will work just fine for the steam. Whatever you do, don't rely on a flow meter, that is, some device that produces a signal representing flow, unless it's a Coriolis meter, and even then make sure it's been tested. Flow meters are not approved by any test code (PTC-19.5, MFC-3M). Acoustic (or ultrasonic) flow meters are OK in some cases, but not in yours.