MES & PSG Feedwater Quality Requirements

Introduction

Mueller Multiple Effect Stills (MES) and Pure Steam Generators (PSG) require specific feedwater quality to ensure the longevity of product quality and the operating unit. Operation outside of these parameters may result in the product water or steam exceeding the requirements for Water for Injection (WFI) or Pure Steam. Other feedwater requirements prevent scaling in the unit, minimizing downtime necessary for descaling.

The feedwater requirements are divided below into three categories: Product Quality Concerns, Operational Concerns, and Concerns Specific to Certain Equipment. Product Quality Concerns potentially affect the unit’s ability to make WFI-quality water or Pure steam-quality steam. These feedwater parameters should be treated as quality-critical and should never be exceeded.

Operational concerns affect the unit's ability to operate as intended. Some of these parameters affect the unit's scaling, which can limit heat transfer and limit the unit's capacity until it is satisfactorily chemically cleaned.

The last category is Concerns Specific to Certain Equipment. These apply only to MESs or PSGs, but not both, and may not apply to all applications. These concerns can generally be designed around during the initial detailed design. They should be brought to the attention of your Regional Sales Manager during the quote stage of any new MES or PSG.

Product Quality Concerns

Amines – Amines, which are an ammonia derivative, are not removed by distillation. They will flash overhead with the pure steam, concentrating slightly in the pure steam or WFI. Even small amounts of amines can lead to high conductivity in the product steam or water, so the limit on amines is near zero.

Conductivity – Conductivity, or the ability of the water to conduct electricity, can come from many sources. Depending on the boiling point of these conductivity-causing substances, they can either boil with the pure steam or stay behind in the blowdown. Because of this, it is not simple to determine whether conductivity in the feedwater will be concentrated or reduced in the product steam or water. The United States Pharmacopeia (USP) WFI limit for conductivity is 1.3µS/cm. Mueller MESs, and optionally Mueller PSGs, are equipped with a feedwater conductivity warning that can be set from 0.0-2.0 µS/cm.

Total Organic Carbon (TOC)—Like conductivity, TOC can have many different sources, some boiling with water and some staying liquid and remaining in the blowdown. Again, it is not simple to determine whether an MES or PSG will concentrate or reduce TOC. Many of our customers feed their MES or PSG with USP Purified Water, which has a limit of 0.5 mg/L, matching the USP WFI requirement for TOC.

Endotoxin – Mueller MESs and PSGs guarantee a minimum of 3-log endotoxin reduction from the feedwater to the product. Since the USP limit for endotoxin in WFI is < 0.25 EU/mL, then the feedwater must be < 250 EU/mL to ensure compliance based on the guaranteed 3-log reduction. While it is not as common as it historically has been, it is possible to do an endotoxin spike test of your MES or PSG before shipment. The feedwater is spiked with endotoxin in this test, and the feedwater and product water samples are sent to a third-party lab for testing. This test aims to prove the 3-log reduction from feedwater to product water and that the product water meets the USP requirement for endotoxin in WFI.

Microbial Loading – The distillation process will kill microbes, so the feedwater has no specific limit. Many customers feed MESs and PSGs with USP Purified Water, which would have a 100 CFU/mL limit.

Operational Concerns

Total Hardness – Hardness in the feedwater will lead to scaling of the MES or PSG over time. Scaling is generally removable with chemical cleaning, but it will affect the heat transfer capabilities of the unit while it is present. Because of this, Mueller requires < 1 ppm total hardness in the feedwater.

Silica – Silica is another source of scaling for the unit; however, silica scale is much harder to remove. Aggressive chemical cleaning is generally needed to remove silica scale. Because of this, Mueller requires < 1 ppm silica in the feedwater.

Chlorine and Chlorides – Chlorine and chlorides in the feedwater (or in the coolant), can lead to stress corrosion cracking at the high operating temperatures of an MES or PSG. No “safe” amount of chlorine or chlorides is allowed in the water, so the level should be as close to zero as practically possible.

Specific to Certain Equipment

Non-Condensable Gasses – Some users of PSGs are required to comply with European Standard EN285, which gives specific guidelines on dryness, non-condensable, and superheating for pure steam. Mueller PSGs, as a base unit with no options, will comply with the dryness and superheat portions of EN285. However, compliance with the non-condensable portion depends entirely on the feedwater coming to the PSG. To comply with EN285, a base unit PSG with no options would need feedwater with < 3.0% non-condensable. If this is not achievable, an optional degasser can be added to the PSG to reduce the non-condensable in the pure steam product.

Temperature – In a Mueller MES, feedwater is preheated in the distillate condenser, which condenses some of the distillate. This serves to reduce the plant steam needed for heating the first effect and serves to reduce the coolant needed in the condenser. Because of this, the feedwater must be at ambient temperature or cooler to effectively cool in the condenser. If it is desired to use hot feedwater for an MES, then the feedwater must be rerouted to bypass the condenser and go directly to the preheaters and first effect. This change will increase the amount of coolant needed in the condenser since the “free cooling” from the feedwater will no longer be available.

If your feedwater exceeds any of these requirements, please contact the Paul Mueller Company service team or your pretreatment vendor to help bring the feedwater back in line. If you are specifying new equipment and anticipate that your feedwater may not meet these requirements, please reach out to your regional sales manager. They will ensure that any deviations are accounted for in the detailed design of your unit.