Paul Mueller Company Academy

MES & PSG Feedwater Quality Requirements

October 10, 2022 | by Karen Reed


Mueller Multiple Effect Stills (MES) and Pure Steam Generators (PSG) require specific feedwater quality to ensure product quality and operating unit longevity. 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.

PyroPure Condenser Graphic

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 ability of the unit to operate as it is intended. Some of these parameters affect scaling of the unit, which can limit heat transfer and limit the capacity of the unit 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 initial detailed design, and 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) – TOC, like conductivity, can have many different sources, some that boil with water and some that stay liquid and remain 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 reduction of endotoxin 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 prior to shipment. In this test, the feedwater is spiked with endotoxin and samples of the feedwater and product water are sent to a third party lab for testing. The aim of this test is to prove the 3-log reduction from feedwater to product water, and to prove that the product water meets the USP requirement for endotoxin in WFI.

Microbial Loading – Microbials will be killed by the distillation process, so there is no specific limit in the feedwater. Many customers feed MESs and PSGs with USP Purified Water, which would have a limit of 100 CFU/mL.

Operational Concerns

Total Hardness – Hardness in the feedwater will lead to scaling of the MES or PSG over time. The scaling is generally removable with a chemical cleaning, but will affect the heat transfer capabilities of the unit while 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. There is no “safe” amount of chlorine or chlorides allowable 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-condensables, and superheat 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-condensables portion is entirely dependent on the feedwater coming to the PSG. In order to comply with EN285, a base unit PSG with no options would need feedwater with < 3.0% non-condensables. If this is not achievable, an optional degasser can be added to the PSG to reduce the non-condensables in the pure steam product.

Temperature – In a Mueller MES, feedwater is preheated in the distillate condenser, using it to condense some of the distillate. This serves to reduce the plant steam needed for heating the first effect, and also serves to reduce the coolant needed in the condenser. Because of this, it is imperative that the feedwater be ambient temperature or cooler so that it will 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 Mueller Biopharm Service or your pretreatment vendor to help bring the feedwater back in line. If you are specifying new equipment and expect your feedwater to fall outside of these requirements, contact your Mueller Regional Sales Manager to ensure that these items are addressed in the detailed design of your unit.

Topics: Pharmaceutical

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