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Differential pressure reference - Pharmaceutical HVAC
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Differential pressure reference

1. Introduction


In this new post of the construction details series, we will discuss the importance of zero reference differential pressure in monitoring cleanrooms and the challenges we face when ensuring compliance with ISO 14644 standards and Annex 1 of the GMP. Recently, I have observed different approaches in various organizations to handling this issue, and this post aims to shed some light on the topic and generate an interesting debate.

2. Differential Pressure Measurement Options


Differential pressure transmitters (for control systems or BMS) or indicators have two inputs, as we know. One positive input is connected to the room with higher pressure, and the other negative input is connected to the room with lower pressure.

Typically, in positively pressurized rooms, the positive input is connected to the room whose pressure we want to measure. Depending on where we connect the negative input, we have the following options:

2.1. Connection to Adjacent Room:


In this option, the positive input is connected to the room we wish to monitor, and the negative input is connected to the adjacent room. This way, we directly visualize the degree of overpressure of one room compared to its surroundings. However, this is not a visible system, and we cannot see the pressure scaling as a whole. Nonetheless, it is not affected by fluctuations from the outside, except in areas directly connected to the surrounding exterior zone. Thus, in the example below, we can see that the differential pressure between Room 3 and Room 1 is +30 Pa.

differential pressure sketch

2.2 Connection to Zero Reference

Here, we will connect the positive input to the room being monitored, but unlike the previous case, the negative input of all rooms will be connected to a common zero reference. In this setup, we can indeed observe the highest and lowest pressure values as a whole and appreciate the scaling between different rooms. In this post, we will delve deeply into this latter case.

differential pressure sketch

3. The challenges of measuring zero reference differential pressure


The first idea that arises when we want to establish a zero reference for differential pressure is to locate it outside the building. This is a commonly appreciated mistake in some pharmaceutical facilities because, in reality, static pressure depends on the current weather conditions. Let’s consider the following example:

differential pressure sketch building

Imagine that we decide to reference the zero point of all cleanrooms to a location outside the building. If the wind speed is 4 m/s, the upwind pressure is 1.0 Pa, and the downwind pressure is -3.24 Pa. The values will be far from zero even if we take the reference to two or more points outside.

In the following graph, we observe the differential pressure graph of 5 rooms with zero reference located outside the building. A storm phenomenon with strong winds coincided with the observed alterations in the room pressures.

Due to the stringent cleanliness requirements, cleanrooms are often constructed under the concept of “box in a box.” That is, cleanrooms are designed as relatively isolated spaces within the building envelope. The interstitial space between the envelope and the cleanroom is therefore the ideal location to place the zero reference for differential pressure.

The measurement of differential pressure must be precise and representative of room pressure, and it must be as stable as possible. Additionally, the influence of areas subject to turbulence, direct air intake or exhaust, personnel movement, or drafts produced by the opening and closing of doors should not cause fluctuations in the readings. This is particularly crucial when performing dynamic control of room differential pressure.

4. Design Considerations

As deduced from the aforementioned, we will place the zero reference in the interstitial space between the cleanroom and the building envelope.

Differential pressure transmitters are typically located in a technical area, in a control panel designated for this purpose, as close as possible to the rooms they monitor. If the number of rooms on the floor is significantly large, control panels must be strategically placed to minimize this distance.

To ensure a common zero reference for all rooms, it is advisable to install a header that runs through the technical area to collect the zero reference from each of these control panels.

This header can have the following characteristics:

  • It should have a diameter typically between DN-70 and DN-100.
  • The construction material can be plastic.
  • It is usually closed at one end and open at the other. To prevent the entry of particles that could clog it, an air filter should be installed at the other end.
  • The location where the circuit is open should be away from ventilation air currents, doors, or machinery that could affect the static pressure of that location.
  • The route should collect all control panels from the entire floor.
  • If there are multiple levels, there should be a header on each level.

5. Conclusions

In conclusion, achieving accurate and stable differential pressure in pharmaceutical cleanrooms is essential for ensuring compliance with industry standards and maintaining optimal cleanliness levels. By carefully considering the placement of zero reference points and implementing appropriate design considerations, such as using interstitial spaces between cleanrooms and building envelopes.

Furthermore, the use of a header to collect zero references from multiple control panels helps ensure consistency and uniformity across all monitored rooms. these design strategies not only enhance the reliability of differential pressure monitoring but also contribute to the overall effectiveness of cleanroom operations.

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