Case History Reducing Methane Emissions From High Bleed Pneumatic Controllers Offshore
Authors: Scalfano David B. 
Published: December 2007

High pressure natural gas is often used in process controllers in the oil and gas production industry. The constant bleed of these controllers emits several hundred standard cubic feet per day of natural gas (95% methane) to the atmosphere. Many new ideas on ways to reduce or eliminate this constant bleed are being developed. This paper reviews one pilot program testing one low bleed product. The product is called the Mizer valve. The product reduced bleed as claimed and operated well initially, but the long term ability of the product, utilizing field gas, to control the processes reliably is currently still under review.

Introduction

A pilot program to evaluate the Mizer Control Valve was performed in the Vermillion 245 Field. The Mizer Valve, manufactured by Quality Machine and Supply Inc. (QMS) in Lafayette, Louisiana, is a low bleed retrofit valve kit for the popular oil field process controllers from Fisher, Norrseal and C.E. Invalco. The Environmental Protection Agency is in the midst of recruiting companies to join their Natural Gas Star Program. The program encourages companies to voluntarily reduce methane emissions by implementing the low bleed controllers and other best management practices. Compressed natural gas (95% methane) is utilized in most offshore pneumatic control systems.

In stock trim, the standard controllers utilize a nozzle and flapper arrangement which continuously vents 5-50 standard cubic feet per hour (SCFH). The mizer valve utilizes a unique nozzle with internal poppet arrangement to reduce bleed rates to a range of 0–8 SCFH. Pre - and post-bleed rates will be discussed later in the report.

The pilot program included two platforms in the Vermillion 245 field. The platforms were Vermillion 246 "D" which has 19 controllers and Vermillion 246 "F" which has 30 controllers. The platforms were selected because of their number of controllers, available fuel gas meter, and volume of sales gas. The desired outcome of the pilot program was to evaluate the operational effectiveness of the mizer retrofit while measuring the actual savings in natural gas. The manufacturer claim gas savings in the 80-95% range. A positive public perception from voluntarily reducing methane air emissions is an added benefit.

Mizer Valve

The Mizer Valve is a retrofit kit to convert existing "high" bleed nozzles in level controllers and pressure controllers commonly used in the oilfield. Existing "high bleed"nozzle/flapper arrangements allow the constant bleed of compressed natural gas to control an automatic valve. As the flapper moves, reacting to pressure or liquid level, and the distance between the flapper and the nozzle changes more gas or less gas is sent to the control valve, signalling it to open or close. Once the process is returned to normal, the distance between the flapper/nozzle is returned to it's normal gap and a constant bleed rate is restored. The constant bleed rate continues until the process needs to make another change. The standard nozzle and flapper have no internal moving parts.

The Mizer control valve comes in two styles, the block nozzle and the plug nozzle. While they appear different, internally they perform the exact same function. The Mizer valve contains an interval poppet that is controlled by movement in the flapper (level controllers) or the beam (pressure controllers). In the steady state mode there is no gas bleed through the Mizer valve. (See Figure 1) This is when the majority of the gas savings takes place. When the liquid level or pressure rises, the flapper or bourdon tube moves imparting a force on the Mizer poppet. Internally the Mizer directs the supply gas to the control valve to open or close the valve.

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